I'm considering purchasing a lightweight pulley, but now I see some places are selling sets of three? What are the other 2 for? Is there a marked advantage to getting the set of 3?

Thanks,
~Paul

the other two are for the steering wheel and the alternator

Just frees up more power as the motor uses less power to move the stock pulleys.

just reduces the weights for the accessory pullies--> less load for the crank--> more efficient power transfer to the wheels.