Hi vijaykrishnan...,

Check the below sample.

HTML

<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
<script type="text/javascript" src="https://ajax.aspnetcdn.com/ajax/jquery.ui/1.8.9/jquery-ui.js"></script>
<link rel="stylesheet" type="text/css" href="https://ajax.aspnetcdn.com/ajax/jquery.ui/1.8.9/themes/start/jquery-ui.css" />
<script type="text/javascript">
    $(function () {
        $("div.accordian").accordion({
            collapsible: true,
            active: false,
            autoHeight: false
        });
    });
</script>
<asp:Repeater ID="rptAccordian" runat="server" OnItemDataBound="OnItemDataBound">
    <ItemTemplate>
        <div class="accordian">
            <h3><a class="" href="#"><%#Eval("Title") %></a></h3>
            <div class="accordian">
                <asp:Repeater ID="rptSubAccordian" runat="server" OnItemDataBound="OnSubItemDataBound">
                    <ItemTemplate>
                        <h3><a class="" href="#"><%#Eval("Level") %></a></h3>
                        <div class="accordian">
                            <asp:Repeater ID="rptSubSubAccordian" runat="server">
                                <ItemTemplate>
                                    <h3><a class="" href="#"><%#Eval("Content") %></a></h3>
                                    <div><%#Eval("ContentElement") %></div>
                                </ItemTemplate>
                            </asp:Repeater>
                        </div>
                        <asp:HiddenField ID="hfLevel" runat="server" Value=' <%# Eval("Level") %>' />
                        <asp:HiddenField ID="hfSubTitle" runat="server" Value=' <%# Eval("Title") %>' />
                    </ItemTemplate>
                </asp:Repeater>
            </div>
            <asp:HiddenField ID="hfTitle" runat="server" Value=' <%# Eval("Title") %>' />
        </div>
    </ItemTemplate>
</asp:Repeater>

Namespaces

C#

using System.Data;
using System.Linq;

VB.Net

Imports System.Data
Imports System.Linq

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        var distinctTitle = (from data in GetData().AsEnumerable()
                                select data["Title"]).Distinct().ToList();
        DataTable dt = new DataTable();
        dt.Columns.Add("Title");
        foreach (var item in distinctTitle)
        {
            dt.Rows.Add(item);
        }
        rptAccordian.DataSource = dt;
        rptAccordian.DataBind();
    }
}

protected void OnItemDataBound(object sender, RepeaterItemEventArgs e)
{
    if (e.Item.ItemType == ListItemType.Item || e.Item.ItemType == ListItemType.AlternatingItem)
    {
        string title = ((HiddenField)e.Item.FindControl("hfTitle")).Value;
        var subSection = (from data in GetData().AsEnumerable()
                            where data["Title"].Equals(title)
                            select new { Title = data["Title"], Level = data["Level"] }).Distinct();

        DataTable dt = new DataTable();
        dt.Columns.Add("Title");
        dt.Columns.Add("Level");
        foreach (var item in subSection)
        {
            dt.Rows.Add(item.Title, item.Level);
        }
        Repeater rptSubAccordian = (Repeater)e.Item.FindControl("rptSubAccordian");
        rptSubAccordian.DataSource = dt;
        rptSubAccordian.DataBind();
    }
}

protected void OnSubItemDataBound(object sender, RepeaterItemEventArgs e)
{
    if (e.Item.ItemType == ListItemType.Item || e.Item.ItemType == ListItemType.AlternatingItem)
    {
        string title = ((HiddenField)e.Item.FindControl("hfSubTitle")).Value;
        string level = ((HiddenField)e.Item.FindControl("hfLevel")).Value;
        var subSubSection = (from data in GetData().AsEnumerable()
                                where data["Level"].Equals(level) && data["Title"].Equals(title)
                                select new { Content = data["Content"], ContentElement = data["ContentElement"] }).Distinct();
        DataTable dt = new DataTable();
        dt.Columns.Add("Content");
        dt.Columns.Add("ContentElement");
        foreach (var item in subSubSection)
        {
            dt.Rows.Add(item.Content, item.ContentElement);
        }
        Repeater rptSubSubAccordian = (Repeater)e.Item.FindControl("rptSubSubAccordian");
        rptSubSubAccordian.DataSource = dt;
        rptSubSubAccordian.DataBind();
    }
}

private static DataTable GetData()
{
    DataTable dt = new DataTable();
    dt.Columns.AddRange(new DataColumn[]
    {
            new DataColumn("Title"),
            new DataColumn("Level"),
            new DataColumn("Content"),
            new DataColumn("ContentElement")
    });
    dt.Rows.Add("Microsoft 365 Certification", "FUNDAMENTALS", "MICROSOFT 365 CERTIFIED FUNDAMENTALS", "<p>Course Details Goes Here</p>");
    dt.Rows.Add("Microsoft 365 Certification", "FUNDAMENTALS", "MICROSOFT CERTIFIED DYNAMIC 365 FUNDAMENTALS", "<p>Course Details Goes Here</p>");
    dt.Rows.Add("Microsoft 365 Certification", "ASSOCIATE LEVEL", "MICROSOFT 365 CERTIFIED MODERN DESKTOP ADMINISTRATOR ASSOCIATE", "<p>Course Details Goes Here</p>");
    dt.Rows.Add("Microsoft 365 Certification", "ASSOCIATE LEVEL", "MICROSOFT 365 CERTIFIED MESSAGING ADMINISTRATOR ASSOCIATE", "<p>Course Details Goes Here</p>");
    dt.Rows.Add("Microsoft 365 Certification", "ASSOCIATE LEVEL", "MICROSOFT 365 CERTIFIED SECURITY ADMINISTRATOR ASSOCIATE", "<p>Course Details Goes Here</p>");
    dt.Rows.Add("Microsoft 365 Certification", "ASSOCIATE LEVEL", "MICROSOFT 365 CERTIFIED TEAMS ADMINISTRATOR ASSOCIATE", "<p>Course Details Goes Here</p>");
    dt.Rows.Add("Microsoft 365 Certification", "EXPERT LEVEL", "MICROSOFT 365 CERTIFIED ENTERPRISE ADMINISTRATOR EXPERT", "<p>Course Details Goes Here</p>");
    dt.Rows.Add("Microsoft MCSE Certification", "ASSOCIATE LEVEL", "MICROSOFT MCSA CERTIFICATION", "<p>Course Details Goes Here</p>");
    return dt;
}

VB.Net

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
    If Not Me.IsPostBack Then
        Dim distinctTitle = (From data In GetData().AsEnumerable()
                                Select data("Title")).Distinct().ToList()
        Dim dt As DataTable = New DataTable()
        dt.Columns.Add("Title")
        For Each item In distinctTitle
            dt.Rows.Add(item)
        Next
        rptAccordian.DataSource = dt
        rptAccordian.DataBind()
    End If
End Sub

Protected Sub OnItemDataBound(ByVal sender As Object, ByVal e As RepeaterItemEventArgs)
    If e.Item.ItemType = ListItemType.Item OrElse e.Item.ItemType = ListItemType.AlternatingItem Then
        Dim title As String = (CType(e.Item.FindControl("hfTitle"), HiddenField)).Value
        Dim subSection = (From data In GetData().AsEnumerable()
                            Where data("Title").Equals(title)
                            Select New With {Key _
                            .Title = data("Title"), Key _
                            .Level = data("Level")
                        }).Distinct()
        Dim dt As DataTable = New DataTable()
        dt.Columns.Add("Title")
        dt.Columns.Add("Level")
        For Each item In subSection
            dt.Rows.Add(item.Title, item.Level)
        Next
        Dim rptSubAccordian As Repeater = CType(e.Item.FindControl("rptSubAccordian"), Repeater)
        rptSubAccordian.DataSource = dt
        rptSubAccordian.DataBind()
    End If
End Sub

Protected Sub OnSubItemDataBound(ByVal sender As Object, ByVal e As RepeaterItemEventArgs)
    If e.Item.ItemType = ListItemType.Item OrElse e.Item.ItemType = ListItemType.AlternatingItem Then
        Dim title As String = (CType(e.Item.FindControl("hfSubTitle"), HiddenField)).Value
        Dim level As String = (CType(e.Item.FindControl("hfLevel"), HiddenField)).Value
        Dim subSubSection = (From data In GetData().AsEnumerable()
                                Where data("Level").Equals(level) AndAlso data("Title").Equals(title)
                                Select New With {Key _
            .Content = data("Content"), Key _
            .ContentElement = data("ContentElement")
        }).Distinct()
        Dim dt As DataTable = New DataTable()
        dt.Columns.Add("Content")
        dt.Columns.Add("ContentElement")
        For Each item In subSubSection
            dt.Rows.Add(item.Content, item.ContentElement)
        Next
        Dim rptSubSubAccordian As Repeater = CType(e.Item.FindControl("rptSubSubAccordian"), Repeater)
        rptSubSubAccordian.DataSource = dt
        rptSubSubAccordian.DataBind()
    End If
End Sub

Private Shared Function GetData() As DataTable
    Dim dt As DataTable = New DataTable()
    dt.Columns.AddRange(New DataColumn() {
                        New DataColumn("Title"),
                        New DataColumn("Level"),
                        New DataColumn("Content"),
                        New DataColumn("ContentElement")})
    dt.Rows.Add("Microsoft 365 Certification", "FUNDAMENTALS", "MICROSOFT 365 CERTIFIED FUNDAMENTALS", "<p>Course Details Goes Here</p>")
    dt.Rows.Add("Microsoft 365 Certification", "FUNDAMENTALS", "MICROSOFT CERTIFIED DYNAMIC 365 FUNDAMENTALS", "<p>Course Details Goes Here</p>")
    dt.Rows.Add("Microsoft 365 Certification", "ASSOCIATE LEVEL", "MICROSOFT 365 CERTIFIED MODERN DESKTOP ADMINISTRATOR ASSOCIATE", "<p>Course Details Goes Here</p>")
    dt.Rows.Add("Microsoft 365 Certification", "ASSOCIATE LEVEL", "MICROSOFT 365 CERTIFIED MESSAGING ADMINISTRATOR ASSOCIATE", "<p>Course Details Goes Here</p>")
    dt.Rows.Add("Microsoft 365 Certification", "ASSOCIATE LEVEL", "MICROSOFT 365 CERTIFIED SECURITY ADMINISTRATOR ASSOCIATE", "<p>Course Details Goes Here</p>")
    dt.Rows.Add("Microsoft 365 Certification", "ASSOCIATE LEVEL", "MICROSOFT 365 CERTIFIED TEAMS ADMINISTRATOR ASSOCIATE", "<p>Course Details Goes Here</p>")
    dt.Rows.Add("Microsoft 365 Certification", "EXPERT LEVEL", "MICROSOFT 365 CERTIFIED ENTERPRISE ADMINISTRATOR EXPERT", "<p>Course Details Goes Here</p>")
    dt.Rows.Add("Microsoft MCSE Certification", "ASSOCIATE LEVEL", "MICROSOFT MCSA CERTIFICATION", "<p>Course Details Goes Here</p>")
    Return dt
End Function

Screenshot