Getting data from AJAX

Hi everyone.
I am using the modalpopup AJAX control.  User enters data into textboxes and dropdownlists, and when user clicks on OK button, I need to retrieve the data, do some operations, and refresh the parent page where the modalpopup originated by clicking on a link button.
I have a javascript function onOk(), but I do not know how I can get a textbox's data for example.  I tried  $get('TextBox3').Text, but it doesn't work.
If I do alert('Test'), it works. Please shine some light here.
Thanks.
 
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