The 3 drain overflow is known as a "Bean Animal" the main drain is a full siphon the 2nd drain is a open channel and only has a trickle of water entering it, the 3rd drain is emergency only and remains dry under normal operation.The 1000 gph rating would be for the main drain.
This video explains it well
https://www.youtube.com/watch?v=RJQM...cMarineSystems