Is it really a 1000w LED? Are we talking "simulated" wattage or what it actually pulls from the wall? Most LEDs should be rated by what they pull from the wall, i.e the wattage of the driver. Most lights that are 1000w are actualy anywhere from 45-150w depending on whatever the manufacturer wants to say.
That being said, 3 plants in a 2x2-3x3-2x4 space with that should be okay 18-24 inches. It'll be "ok".
Best of luck!