For only playing for one week, that is pretty good. Though three-stocking a CPU is not something amazing, as they are stupid. You abused stupid CPU qualities, like not using an up-B if used already (first stock), almost never teching, having a very predictable recovery (second stock), dead easy to combo / don't get out of some combos well, and walk into dead-obveous traps (third stock).
I would say at that point, play people online on servers or P2P. That would be the best way for you to improve. You won't get much farther against CPU's, and there definitely are some people near your skill level on servers. Playing against humans is much better than playing against CPU's. Also, if you play against someone better than you, learn from them.
Keep up the good work, and don't give in to the dark side of the smash world (i.e. Brawl).