how to add server side popup window

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ASP.NET

SQL-Server-2008R2

i am writing popup window code(server side) on button click.
it showing popup window with four asp:buttons,but when i select one button and click on it it won't redirect me to desire page.
what is the problem?
code:-

<script>
function ShowPopup() {
        $(function () {
          
            $("#dialog").dialog({
                title: "SELECT SUBJECT",
                width:400,
                buttons: {
                    Close: function () {
                        $(this).dialog('close');
                    }
                },
               
            });
        });
    };
</script>

<pre lang="xml"><div id="dialog" style="display: none" runat="server">
    <asp:Button ID="Button3" runat="server" Text="Button" Height="50px"
        Width="150px" onclick="Button3_Click" /> &nbsp&nbsp&nbsp
        <asp:Button ID="Button4" runat="server" Text="Button" Height="50px"
        Width="150px" onclick="Button4_Click" /> &nbsp&nbsp&nbsp <br />
        <asp:Button ID="Button5" runat="server" Text="Button" Height="50px"
        Width="150px" onclick="Button5_Click" /> &nbsp&nbsp&nbsp
        <asp:Button ID="Button6" runat="server" Text="Button" Height="50px"
        Width="150px" onclick="Button6_Click" />
    </div>
aspx.cs:-
protected void Button2_Click(object sender, EventArgs e)
{
.
.
.

 reader2.Read();
        
        Button3.Text = reader2["subject1"].ToString();
        Button4.Text = reader2["subject2"].ToString();
        Button5.Text = reader2["subject3"].ToString();
        Button6.Text = reader2["subject4"].ToString();
        reader2.Close();
        ClientScript.RegisterStartupScript(this.GetType(), "Popup", "ShowPopup()", true);
}
 protected void Button3_Click(object sender, EventArgs e)
    {
        
        Response.Redirect("attendance.aspx");
    }

this is my code,popup is showing with four buttons but when i click on button3 it is not redirecting me.