CMS Can someone help me set up this grid
 

I need left sidebar at 120px, liquid center column, right column at 275px

When I look through the different grid layouts and settings i can only find where to choose a fixed width for the sidebar, the other two columns always seem to be based on percentage only (ex: 60/40 or 70/30)

I want both left and right columns fixed and the center column to just use whatever space is left.

Thanks

p.s. I went to the "3 column fixed-liquid-fixed" grid and it shows a side bar and one column at 100 percent. Yet when you go to edit a layout using this grid it shows 3 columns. I dont understand how you edit the right column width or where it even came from if that is not indicated in the settings for that grid.