How to sum up fields using C3 and ASp.net

How do i sum  up 3 fields while inserting into the database. I have 3 fields
ext.1
ext.2
ext.3
total ext.
I need the total ext to sum up the three previews fields and pop up automatically on mouse over.
total ext = ext.1 + ext.2 + ext.3

any suggestion?
shpresaAsked:
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leakim971PluritechnicianCommented:
window.onload = function() {
document.getElementById("popupLink").onmouseover = function() {
  var v1 = document.getElementById("<%= TextBox1.ClientID %>").value;
  var v2 = document.getElementById("<%= TextBox2.ClientID %>").value;
  var v3 = document.getElementById("<%= TextBox3.ClientID %>").value;
  var t0 = v1*1 + v2*1 + v3*1;
  showPopup( t0 );
}
}

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