I've run Workbench 3.1 on several Kickstart 3.0 A1200s over the years without a single problem. I don't think I ever even used Workbench 3.0 at all. It is 100% compatible, and probably improved over 3.0, even on 3.0 ROMs. It is only certain software that requires Kickstart 3.1, as well as Workbench 3.5 and 3.9 of course.