As to the question of adding multiple EVSEs, by "output" of the Gateway you mean the PW side? Then without further controls, if the grid fails while your EVs are charging, they will happily continue to charge until your PWs run out. Even with (5) PWs, the EV may have a bigger battery. An argument for powering from the grid side of the Gateway (or otherwise automatically killing power to them when the grid is out, perhaps with a manual override).
Loads on the PW side of the Gateway are restricted to 200A, or less if the grid supply to the Gateway is smaller. So you would need to do a load calc to see if you have capacity to add (3) 60A EVSEs. What category to put an EVSE in for a residential load calc (i.e. what factor to use on the load before summing) is a little unclear to me, but they would count at least at 40%, or 72A out of the 200A (or less) available.
The same calculations apply on the grid side of the Gateway, but the available power may be higher, depending on the details of the service and feeder.
Cheers, Wayne