Page layout sidebar-content-sidebar?

This related to other WPMU themes, too, not just the Farms....
Total WPMU newbie here, but not new to WP. Most of my customers prefer a three column layout, sidebar-content-sidebar. I've looked at a few of the WPMU themes and don't seem to find it as a page layout option. I may be missing it entirely, but the option just doesn't seem to be where I usually find it from other companies. Is it built into some themes, but not others? If so, is there a list of 3-column themes? If it isn't built in to any themes, I don't suppose there's an "easy" way to add?