With those seatbelts, running three trains would be a moot point. With only two, the second train is almost always finished unloading by the time the first train is ready to launch. Adding the a third wouldn't increase capacity much if at all.
I'm definately not knocking the crew. Last time I rode, they were going very fast; it's just those seatbelt buckles suck. Most of the time it's hard to get them to snap closed even with slack in the belt, let alone if the rider is larger.
I remember checking bars with the old restraints, and it took only about 25-30 seconds from gate-open to launch most of the time.