How do I pair my Powertap G3 hub speed sensor (power and cadence work ok)

I do not think that you can or should as Zwift will use you power, weight etc to figure out have fast you are going in the game. 

I think that speed is only used when you do not have a power meter

The speed your wheel is turning is irrelevant, as Zwift works out your “speed” due to your weight, height, power, cadence, virtual gradient, virtual air pressure, virtual kit etc.