Required field validators not working on button click

I have these validators for two text boxes and a dropdown list:
<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="txt1" ErrorMessage="Required Field." ValidationGroup="Submit" EnableClientScript="false"></asp:RequiredFieldValidator> <asp:RequiredFieldValidator ID="RequiredFieldValidator2" runat="server" ControlToValidate="txt2" ErrorMessage="Required Field." ValidationGroup="Submit" EnableClientScript="false"></asp:RequiredFieldValidator> <asp:RequiredFieldValidator ID="RequiredFieldValidator3" runat="server" ControlToValidate="ddl1" ErrorMessage="Required Field." ValidationGroup="Submit" EnableClientSc ...