Yes, look at the write up on one of the breaker web sites., All 3 leads go through the current transformer and get summed.
As for what it takes to trip a GFCI, when the fault current is in that 4-5 MA range it is just waiting fir any little deviation to push it over the edge. You might be able to see something going from each leg to ground with an ohm meter but it is usually easier to just start eliminating loads and see if one is the culprit. I have worked on circuits that split out and went several ways. It was really hard to find the bad leg but I usually found one. It could be as simple as ants in a motor or water in a box.
