How to bind multiple DropDownList if Not IsPostBack on PageLoad using jQuery in ASP.Net

Here i have created sample that full fill your requirement.

HTML

<center>
    <table>
        <tr>
            <td>
                Countries:
            </td>
            <td>
                <asp:DropDownList ID="ddlCustomers" runat="server">
                </asp:DropDownList>
            </td>
        </tr>
        <tr>
            <td>
                States:
            </td>
            <td>
                <asp:DropDownList ID="ddlStates" runat="server">
                </asp:DropDownList>
            </td>
        </tr>
        <tr>
            <td>
                Cities:
            </td>
            <td>
                <asp:DropDownList ID="ddlCities" runat="server">
                </asp:DropDownList>
            </td>
        </tr>
    </table>
    <br />
    <asp:Button ID="Button1" Text="Save" runat="server" OnClick="Save" />
</center>
<div>
    <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
    <script type="text/javascript">
        $(function () {
            // if not postback then binds dropdownlist
            if (!isPostBack) {
                var ddlCountries = $("[id*=ddlCustomers]");
                var ddlStates = $("[id*=ddlStates]");
                var ddlCities = $("[id*=ddlCities]");
                //Populate Country DropDown
                AjaxCall('GetCountries', ddlCountries);
                //Populate State DropDown
                AjaxCall('GetStates', ddlStates);
                //Populate City DropDown
                AjaxCall('GetCities', ddlCities);
            }
        });

        function OnSuccess(response, ddl) {
            ddl.empty().append('<option selected="selected" value="0">Please select</option>');
            $.each(response.d, function () {
                ddl.append($("<option></option>").val(this['Value']).html(this['Text']));
            });
        }

        function AjaxCall(method, ddl) {
            $.ajax({
                type: "POST",
                url: "CS.aspx/" + method,
                data: '{}',
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                success: function (r) {
                    OnSuccess(r, ddl);
                }
            });
        }
    </script>
</div>

Code

protected void Page_Load(object sender, EventArgs e)
{
    string script;
    if (IsPostBack)
    {
        script = "var isPostBack = true;";
    }
    else
    {
        script = "var isPostBack = false;";
    }
    Page.ClientScript.RegisterStartupScript(GetType(), "IsPostBack", script, true);
}

static string constr = ConfigurationManager.ConnectionStrings["constr"].ConnectionString;
[WebMethod]
public static List<ListItem> GetCountries()
{
    List<ListItem> countries = ListItems("SELECT CountryId,CountryName FROM Countries");
    return countries;
}

[WebMethod]
public static List<ListItem> GetStates()
{
    List<ListItem> states = ListItems("SELECT StateId,StateName FROM States");
    return states;
}

[WebMethod]
public static List<ListItem> GetCities()
{
    List<ListItem> cities = ListItems("SELECT CityId,CityName FROM Cities");
    return cities;
}

public static List<ListItem> ListItems(string query)
{
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand(query))
        {
            List<ListItem> items = new List<ListItem>();
            cmd.CommandType = CommandType.Text;
            cmd.Connection = con;
            con.Open();
            using (SqlDataReader sdr = cmd.ExecuteReader())
            {
                while (sdr.Read())
                {
                    items.Add(new ListItem
                    {
                        Value = sdr[0].ToString(), //Value Field(ID)
                        Text = sdr[1].ToString() //Text Field(Name)
                    });
                }
            }
            con.Close();
            return items;
        }
    }
}

protected void Save(object sender, EventArgs e)
{
        
}

Screenshot