Usually you would just make your page 960px wide and give everything a 20px margin right.
If you want to do it the way you are trying, you would have to add a little hand coding to treat every third column differently.
This would be done by adding a counter to keep track of the number of divs like:
right above the DO loop line for your repeat region.
Then in the line right above the WHILE line that ends your loop you would add:
Then you can add code that would only appear every third row like:
<?php echo(($row_count%3==0)?' style="margin-right=0;"':' style="margin-right:20px"'); ?>