Every Razzie nomination deserved it really, but I thought that GL should have got one for Worst Director, easily. If anyone doubted it after EPII, they were just kidding themselves - Lucas is a poor director, we're just lucky that ANH turned out so brilliantly.
HC is the supporting actor because he's not the big name on the bill - Ewan McGregor and Ian McDiarmid is. That's why GL tried to get Ian nominated for Best Actor at the Academy Awards. I'm not sure how much screen time Anakin and Obi get in EPIII, but that might also have a bit to do with it.