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I assume that you at least know the horsepower ratings of the motors. The key thing to know is that 746 watts equals one horsepower. So if you have a 2 hp motor, you should be able to operate it with a 1500 watt generator. Except you can't, due to power factor, inefficiency and exaggerated generator ratings. So if you double the generator size you should be OK. So a 3 kw generator should have no problem driving a 2 hp motor. Now if you really need the amperage, say to size the wires, this gets more complicated. I assume that the motors are 208V delta. So for our 2 hp example: HP per phase: 2 hp / 3 = .67 hp Watts per phase: .67 / 746 = 500 W volt-amps per phase:
(If the power factor is stated on the data plate, use that figure, otherwise assume .7)
VA = watts / PF = 500 / .7 = 710 volt-amps amps per phase:
We have to pretend that the motor is a wye-wound motor, so we can use the phase to neutral voltage instead of the phase to phase voltage. (Never mind that there is no neutral.) The phase to neutral voltage is the phase to phase voltage divided by the square root of three. So for 208 volts phase to phase, the phase to neutral voltage is 120 volts.
So amps = 710 volt-amps / 120 volts = 6 amps. I wouldn't worry too much about starting currents, as long as you've sized your generator and wiring conservatively you should be OK.
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