I have 2 10's and 1 8". they are all (svc4ohm). How big of a mono amp do I need?

you need an amp rated at 450 watts rms @ 1 ohm...well since the final load of the subs is 1.34 ohms then I guess you can get an amp that sends 500 watts rms @ 1 ohm...check the link for all the wiring options