Calculating power with three phase?
The 100 amp, 120/208 volts, 3-phase rating tells you that the 120 volts is available from each of three 120 volt line to-neutral circuits. Each of those three circuits provide 100 x 120 = 12,000 watts for 36,000 watts total.

You mentioned 0.8pf, but conventional incandescent fixtures would have 1.0 pf. If you have been told that the fixtures are LED fixtures with 0.8 pf, the power available would be 0.8 x 36,000 watts.

If 100 amp, 120/208 volts, 3-phase is available from a single disconnect it would be a 3-phase disconnect supplying the three circuits described.