I'm putting together a 3" exhuast.
do the resonator matters? I didn't get/use one...

 

A resonator, to the best of my knowledge, is not required for any vehicle in the States. If your vehicle is illegally loud as a result, then they [The Man / inspection points] will probably want you to have one.

Only items required for an exhaust on any emissions-controlled vehicle are a catalytic converter, and at least one muffler.