Just tore off my front and rear doors and both doors have a 5 3/4" opening. Meaning that most 6 3/4" speakers will fit. 6 1/2 will fit also although some will be too small and you will need to make an adapter ring of some sort. GM uses a dumb clip design so any aftermarket speaker you install will require you to drill into the door, unless you get a gm adapter (if they even make one for the h3)

hope this helps.

 

I'd check Crutchfield for the adapters .. they are usually pretty good with that stuff.

 

easy and cheaper to make a ring out of mdf. or just screw into the door.