There are 11 weapons and more than 3 levels per weapon. However you only have to level your weapons to level 3 a piece. You can get this easily, any time you're given a weapon that is below level 3 and you have ammo for it, make sure you use it. A few weapons (groovetron) should be used and then switched off of, but for the most part just be aware of the levels of what weapons you currently are given for a level.
You should have this before you finish the game.