Hi
Here is some more information about column width , colwidth=”20″ parameter value sets a default column width for Left ,right1, right2 etc columns. When Left & right etc uses a column width of 20%, and main body will use the remaining area from the total width, so if you have say 3 columns, main body will be given 100-(20*3)=40%.
For your current requirement, you can change columnwidth as per this code :
<blocks name=”middle” colwidth=”6″ fixheight=”1″>
<block name=”inset1″>left</block>
<block name=”right1″ width=”12″>right</block>
<block name=”right2″>right2</block>
</blocks>
For more information refer this link : http://wiki.joomlart.com/wiki/JA_T3_Framework_2/Guides