<script type="text/javascript">
        
        function Show() { $('#hide').attr('value', $(this).attr('value')); }
</script>

<asp:Button Text="3" runat="server" ID="btn1" class ="btnClass" OnClientClick="Show();" />
<asp:HiddenField runat="server" ID="hide"/>

<script type="text/javascript">
        $(document).ready( function {
        $('.btnClass').on('click', function() { $('#hide').attr('value', $(this).attr('value')); })})
</script>

<script type="text/javascript">
        
        function Show(btn) { $('#hide').attr('value', $(btn).attr('value')); }
</script>

<asp:Button Text="3" runat="server" ID="btn1" class ="btnClass" OnClientClick="Show(this);" />
<asp:HiddenField runat="server" ID="hide"/>