I have unlocked everything, but still don't have 3star ranking on any cup
For time trial - watch the staff ghost, pay special attention to the kart/bike they are using, where they use their mushrooms, and what line they take.
Use the magicruiser for Sherbert land and the beach level with the crabs. On sherbert land, the entire course is offroad, and the magicruiser has the best offroad stats in the game. On the beach level, take the magicruiser and drive in the water pretty much all the time - much faster
Use manual. That's all I have to say about that.
If you have any questions about specific courses, let me know