Uworld is king for M3 (Except for Family Med where I think most people in my class used ExamGuru). Zanki can be useful for certain shelves where your study time can be limited, like surgery or OB. The idea is the more Uworld you do, the better off you'll be. They added 500+ questions since the start of my M3 (and I just took CK on Friday), and a lot of it is repetition.