3 phase circuit, 3 x single-phase (hot-hot) loads: what ampacity?
3 times 100A at 120V means you've got 36KW coming into your fuse board.

3 times 57.7A at 208V means you've got 36KW going out of your fuse board.

Assuming your load is balanced, anything more than 57.7A on the load side will be more than 100A on the supply side.

Where I work, we step down the wiring and the circuit breakers so the end points break before the supply points, so for 208V circuits we'd be looking at a 3 phase (3 pole) 50A circuit breaker to break all three circuits if there is an overload on one, or 3 single phase 50A circuit breakers if you can allow single phase loads to fail independantly