I think you should play all three, so you can get the experience you will need to be the player you want to be. The first thing you should learn is BRM. It's not hard to learn what BRM is, but it is hard to use it and to be in control of it at all times. If you do, you will have a BR to play with and be able to build it up. If you don't, I'm sorry to say that more than likely you will lose more than you win, and your BR will go down fast or slow. That depends on the buy ins you use for MTTs, cash games, and sit n gos you want to play. If you let us know how much you plan to start a BR with, then we can tell you the best games you should play according to that. This will go a long with your BRM. Take your time because there is no fast way to learn all you need, and to be real, there is no end to it, so always keep an open mind, and use the advice you get here at the table. I did from the start here, and it worked.