it's not really necessary to fly a 50 for 3d, i learnt loops, rolls, inverted, backwards and lots of basic 3d with a 30, it teaches you to be a better pilot because you have to manage the pitch more carefully with the 30's, the 50's will ultimately 3d better and allow you to come along a bit faster because of the greatly enhanced power to weight ratio but a 30 is still a great trainer even for 3d.

Gareth