3 GPM is OK, except maybe when somebody is showering and another faucet gets turned on full. That can cause a flow drop and a yell from the shower! But in most cases the lack of adequate water flow in an RV is NOT caused by insufficient pump pressure - it is typically something else.
If you are replacing yours, consider a variable flow pump. They are more costly but smoother in operation. Some of them require up to 10 amps of 12v power, though, so may be too much for your existing pump control relay and wiring.