Train all your 3 fingers. Now I use 3 fingers without thinking....i'm very proud. After you have learned to quite good with 3 fingers you just train your speed. Playing Iron Maiden songs is good, but really, whatever you play it is good. Its important to train your cordination.
I remember the first time I saw "Run To The Hills"...I tought that was impossible, now, 1 year later (i started to use 3 fingers 1 year ago) I can sleep while playing it....its not really that hard. And you can play it with 2 fingers to, my bass teacher can galopp faster then me and he uses 2 fingers...it looks very cool...
