When you click a submit button, it will add the name of that button to the $_POST vars. So if you had 3 submit buttons with the names of A, B, and C, and when you click on button A, only that variable will be set in the post vars along with the "checkboxes" that were checked. So depending on which button was pressed, you can test which one was actually clicked by checking if that post var isset() or you can even set the names of the buttons the same and then determine which button was pressed by the value of it.
For example,
<input type=submit name=action value=add> <input type=submit name=action value=delete> <input type=submit name=action value=cancel>
if someone clicked on the add button, you can determine it by this.
if( $_POST["action"] == 'add' )
{ do a process }
Hope this helps.