Hello
Not a really clear question but:
If you want javascript in content page then:
call
Page.ClientScriptManager.RegistgerClientScriptBlock
in content page
And if your problem is Sum of tree textboxes then you need to Pars their value to int or double or ...
for example
int number1 = 0;
int number2 = 0;
int number3 = 0;
int.TryParse(OSIInterstFromBank.Text, out number1);
int.TryParse(txtOSITDSFromBank.Text, out number2);
...