Cory and Zerfino are probably too old to get any better and Bruze could get a little better but not much because his potential is only a role player.
Players get better by training. There is a lot too explain about training, I would strongly suggest reading about it in the Guide to Buzzerbeaters here:
(http://www.buzzerbeater.com/community/forum/read.aspx?thr...)You also should read the Draft section in the Guide as well. That is where you can get your young players to train.