Yes, DC would only test the general overload thermal trip and DC testing would certainly take its toll on the contacts. I wasn't suggesting running 5 times the In current rating of a C curve breaker to test the mag trip (hence the stopwatch suggestion).

A 3-phase MCB should open all 3 poles if the current through any single pole reaches the tripping curve so if I was testing 3-phase MCB's I'd probably test each pole separately. Testing them simultaneously in series wouldn't guarantee all 3 poles are tripping correctly.