function add(){
var txtMoney1=document.getElementById("txtMoney1").value;
var txtMoney2=document.getElementById("txtMoney2").value;
var txtMoney3=document.getElementById("txtMoney3").value;
var sum=parseFloat(txtMoney1)+parseFloat(txtMoney2)+parseFloat(txtMoney3);
document.getElementById("txtMoney").value=sum;
}
-->
</script>
<body>
<asp:textbox id="txtMoney1" runat="server" onchange="add();">0 </asp:textbox>