Find which DropDownList Value is Changed in ASP.Net

Try this..

CSS:

<style type="text/css">
        .ShowPopup
        {
            filter: alpha(opacity:50);
            background-color: Black;
            border: 1px solid #fff;
            width: 250px;
            height: 250px;
        }
    </style>

HTML :

<form id="form1" runat="server">
    <UC:ToolkitScriptManager runat="server">
    </UC:ToolkitScriptManager>
    <div>
        <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true" OnSelectedIndexChanged="FindDropdownList">
            <asp:ListItem Text="EXCELLENT" Value="1" />
            <asp:ListItem Text="VERY GOOD" Value="2" />
            <asp:ListItem Text="GOOD" Value="3" />
            <asp:ListItem Text="FAIR" Value="4" />
            <asp:ListItem Text="POOR" Value="5" />
        </asp:DropDownList>
        <asp:DropDownList ID="DropDownList2" runat="server" AutoPostBack="true" OnSelectedIndexChanged="FindDropdownList">
            <asp:ListItem Text="EXCELLENT" Value="1" />
            <asp:ListItem Text="VERY GOOD" Value="2" />
            <asp:ListItem Text="GOOD" Value="3" />
            <asp:ListItem Text="FAIR" Value="4" />
            <asp:ListItem Text="POOR" Value="5" />
        </asp:DropDownList>
        <asp:DropDownList ID="DropDownList3" runat="server" AutoPostBack="true" OnSelectedIndexChanged="FindDropdownList">
            <asp:ListItem Text="EXCELLENT" Value="1" />
            <asp:ListItem Text="VERY GOOD" Value="2" />
            <asp:ListItem Text="GOOD" Value="3" />
            <asp:ListItem Text="FAIR" Value="4" />
            <asp:ListItem Text="POOR" Value="5" />
        </asp:DropDownList>
    </div>
    <div>
        <asp:Panel ID="pnlPopup" runat="server">
            <asp:Label ID="lblMessage" runat="server" />
            <asp:TextBox ID="txtReason" runat="server" />
            <asp:Button ID="btnCancel" Text="OK" runat="server" />
        </asp:Panel>
    </div>
    <asp:LinkButton ID="lnkfake" runat="server" />
    <UC:ModalPopupExtender ID="mpeShowPopup" runat="server" BehaviorID="mpeShowPopup"
        PopupControlID="pnlPopup" TargetControlID="lnkfake" BackgroundCssClass="ShowPopup"
        CancelControlID="btnCancel">
    </UC:ModalPopupExtender>
    </form>

C#:

 protected void FindDropdownList(object sender, EventArgs e)
    {
        DropDownList dropdownList = (sender as DropDownList);
        txtReason.Text = string.Empty;
        if (dropdownList.SelectedItem.Value == "5")
        {
            lblMessage.Text = dropdownList.ID;
            mpeShowPopup.Show();
        }
    }

Best Regards

Sajid Dhukka