Is a 3-bike rack needed for two bikes?
It depends on how... scratch averse you are for your bikes.

A 4 bike trailer hitch rack by Thule can get 4 bikes on it. If you want to ensure that there is never any pedal to frame contact, then you can get 2 bikes on it.

In general with some wiggling and possibly removing a wheel or two, you can always get as many bikes as advertised on a given rack. Just be aware that there will be some rubbing as you fiddle with the bikes to get them on.