You are correct. The panel is rated 100 amp 3 phase. Each phase is limited by the breaker and the conductor rating. As an example you might measure A = 50a B = 60a and C = 50a. You do not add the total and get 110 amps.
If you were to measure 80 amps per phase, it would be 80 ampere, 3 phase on a 100 ampere panel. And, the utility will charge you for the equivelent of 80 amperes, 3 phase which is more than 80 amperes 1 phase.