AFAIK, AC runs with one of the belts in the motor. when AC is turn on your engine has to turn the regular engine and add AC resistance on it. When you turn it off, you will have all ur power back.

I am also not sure about 3hp. I think it depends on car to car,and mainly on how big the AC is.

I used to have 4cyl MX6. If my AC is on highway, i cannot pass anyone on the road. I would turn it off, pass the guy, get the speed i want and then turn it back on.