What Generator size is needed for 10 hp 480 volt 3 phase motor?

Watts and horsepower are directly related as they are both measurements of power. 1hp = 745.6998 watts. So 3kW = 3000W / 745.6998 W/hp = 4.023hp You need a minimum of 4hp, which will give you a max of 2982W before any losses in your generator. You should probably go with a 4.5 or 5 horse motor.