I’m planning a trip, when would be the best time to make a reservation for the beginning of June? Also how can I ask if the 50 amp is a three or a four prongs?

Sooner the better it's a summer vacation spot .. Dunno about the 50 amp . you talking RV. IF so then yes

Need to book it now and they will have details of site and utilities

Asap

The camping places are not open yet, HARVEY

I would make my reservation as soon as possible and they have adapters at most sites

I would call and ask them now, but it is my understanding you would likely not need a resevration.