in magento how do i change the number of columns based upon specific page layout

In magento i have a problem with category page product display default product per row is set as 3

it is well and good in 2columns with left sidebar and rightsidebar

we move on 3 columns it creates a problem product per row is 3  in 3 column i want to set it to 2

how do i create a solution or rectify. please help me