Sum of Textbox values in FormView question

Hello everyone, can some one help guide me to the right path

I have a FormView with 3 Textbox which display the # of Shares (Integer) I want to add a label control to the FormView and when the FormView loaded, I want this label to get the sum of the 3 TextBoxes.  I am new to ASP.NET so any details instruction is much appreciated.  Thanks in advance. 

 

 

 
...