To a close approximation you just add up the per phase kVA to get the total kVA.

As victor.cherkashi notes, the phase angles of a 120/208V system screw this approximation up; if you only had 120V loading and the kVA numbers were accurate, then the amp reading you would have on leg A would be 15.7kVA / 120 = 131A, and on leg B would be 15.49 / 120 = 129A. Because you have both 120 and 208V loading the calculation is more complicated but would give near the above numbers.

In real life, if you put an ammeter on the phases I'd expect something like 30-40A.

This looks sort of like a residence calc, but not quite; and residential calcs are known to be severely conservative. If the kVA numbers are accurate than the 100A main breaker is overloaded, however there are almost certainly diversity factors to consider.

-Jon