If your not putting a Turbo your probably going to lose power if you put a 3" exhaust ( Is it a full cat back or just the muffler) Your going to loose to much pressure and mess up your air to fuel ratio. You would need to flash your ECU to adjust the air to fuel ration so your going to need a e-management kit. Aldo i hope the civic is stick shift because if your driving an auto your definitely not going to gain anything except for a little better throttle response. Unless you floor the gas each time.