Wavedashing depends on the main you are using. All characters use it a lot, but characters like Luigi and Ice Climbers should normally use it as their primary movement option. (at least that's what I've heard from others) But most characters use them a lot to get on platforms much faster and efficiently, and then it is also one great way to edge guard, by grabbing the ledge at the right time, and rolling back onto the stage at the time that they attempt to grab the ledge so you keep the invincibility so they can't hurt you, and they can't grab onto the ledge until after you are done rolling onto the stage.
SHFFLing is primarily used to approach an opponent if your character allows it. With some characters, SHFFLing isn't that helpful.
L-Canceling is just used to speed up the timing at which you can perform different moves after aerials. You should probably be using this every and any time that you use an aerial.
That's pretty much all I know on those subjects, I'm sure there is more that I don't yet know, or maybe something I got wrong, but for the time being, that should help a little bit I hope!