Hi Vincenzo67,

Refer below example.

HTML

<asp:DropDownList ID="ddlMonths" runat="server">
    <asp:ListItem Text="Select" Value=""></asp:ListItem>
    <asp:ListItem Text="Jan" Value="1"></asp:ListItem>
    <asp:ListItem Text="Feb" Value="2"></asp:ListItem>
    <asp:ListItem Text="Mar" Value="3"></asp:ListItem>
    <asp:ListItem Text="Apr" Value="4"></asp:ListItem>
    <asp:ListItem Text="May" Value="5"></asp:ListItem>
    <asp:ListItem Text="Jun" Value="6"></asp:ListItem>
    <asp:ListItem Text="Jul" Value="7"></asp:ListItem>
    <asp:ListItem Text="Aug" Value="8"></asp:ListItem>
    <asp:ListItem Text="Sep" Value="9"></asp:ListItem>
    <asp:ListItem Text="Oct" Value="10"></asp:ListItem>
    <asp:ListItem Text="Nov" Value="11"></asp:ListItem>
    <asp:ListItem Text="Dec" Value="12"></asp:ListItem>
</asp:DropDownList>

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    DateTime dateTime = new DateTime(2025, 01, 10);
    // List of Months to be display.
    List<ListItem> months = new List<ListItem>();
    for (int i = 1; i <= 3; i++)
    {
        months.Add(ddlMonths.Items.Cast<ListItem>()
                            .Where(x => x.Value.Equals(dateTime.AddMonths(-i).Month.ToString()))
                            .FirstOrDefault());
    }

    for (int i = ddlMonths.Items.Count; i > 1; i--)
    {
        if (!months.Contains(ddlMonths.Items[i - 1]))
        {
            // Remove items from DropDownList.
            ddlMonths.Items.Remove(ddlMonths.Items[i - 1]);
        }
    }
}

VB.Net

Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load
    Dim dateTime As DateTime = New DateTime(2025, 1, 10)
    '//' List of Months to be display.
    Dim months As List(Of ListItem) = New List(Of ListItem)()
    For i As Integer = 1 To 3
        months.Add(ddlMonths.Items.Cast(Of ListItem)().Where(Function(x)
                                                                    Return x.Value.Equals(dateTime.AddMonths(-i).Month.ToString())
                                                                End Function).FirstOrDefault())
    Next

    For i As Integer = ddlMonths.Items.Count To 1 + 1 Step -1
        If Not months.Contains(ddlMonths.Items(i - 1)) Then
            ' Remove items from DropDownList.
            ddlMonths.Items.Remove(ddlMonths.Items(i - 1))
        End If
    Next
End Sub

Screenshot