I actually don't own the game so I might sound ******** here, but why are Level 3 computers better practice than Level 9
Level 9s can read your button inputs prior to you even seeing them on-screen.
This allows them to spotdodge it/roll/perfectshield it. However, this is balanced off by the fact that you can manipulate their AI by exploiting flaws in coming from a ledge (most of the time, if you delay a shorthop, they'll just jump up and you can Fair them back, and overall lacking human common sense.
Level 3s are just stupid, but they don't input read as much (note they somewhat still do though), and they at least attack and shield. They offer a basic AI training dummy to work on spacing and tactics on.
I, however, actually got better fighting Lv .9s.
No, I am not joking. If you WANT to enter the competitive scene, and am completely ignorant of what it's like, go fight a level 9 CPU ditto and you'll learn a lot of the tactics advanced players use.
However, that's why we have sites like Youtube, DailyMotion, NicoVideo, etc. to show us the competitive scene, and Smashboards.
But if you were like me, fight a few level 9s and you'll learn a few things. From there, work on spacing with lv. 3s, and naturally
fight characters that give you a bad match-up like ROB vs. Marth fight players generally better than you.
EDIT: Note that fighting Lv. 9s to see what some of the basic competitive tactics are like only applies to Brawl. Melee CPUs seems like they were just for lulz.