"That Third textbox result should store KEYPRESS EVENT "
I think You can do this on click event.
On key press you can allow user to enter only numbers.
On click on third textbox calculate your result.
<label>First number</label> <asp:TextBox ID="TextBox1" runat="server" onkeypress="return isNumberKey(event)"></asp:TextBox> <br />
<label>Second Number</label> <asp:TextBox ID="TextBox2" runat="server" onkeypress="return isNumberKey(event)" ></asp:TextBox> <br />
<label>Your result</label> <asp:TextBox ID="TextBox3" runat="server" ReadOnly="true" onclick="javascript:check()"></asp:TextBox>
<script type="text/javascript">
function isNumberKey(evt)
{
var charCode = (evt.which) ? evt.which : event.keyCode
if (charCode > 31 && (charCode < 48 || charCode > 57))
return false;
return true;
}
function check()
{
var value = document.getElementById('<%=TextBox1.ClientID%>').value;
var value1 = document.getElementById('<%=TextBox2.ClientID%>').value;
if(value!="" && value1!="")
{
var sum = parseInt(value)+parseInt(value1);
document.getElementById('<%=TextBox3.ClientID%>').value = sum;
}
}
</script>
hope this works !