Ackie Monitor question

Hey yall first time posting so I hope Im in the right section. I purchased a trio of ackies 3 weeks ago, 4 months old from an experienced breeder. I was told they were two females and one male, although I am not experienced enough yet to tell the difference (these are my first monitors). I have had plated lizards, a beardie, many newts, and about three different skink species. They are currently housed in a 48" by 18" by 24" converted aquarium. Man am i impressed, very active and avid feeders and explorers. By far the most exciting to sit and watch. However, I only see two of them out, the third is always burrowed and will never come out when im around. I assume the two always out are the females because they look like clones of each other, although one has established her dominance as queen this past week. "queen" has no fear of me and the other only retreats if i move too fast or get within 6 inches of her. Both come to me when i have crickets though. I'm worried about the third (possibly the male) because i never see him eat. Extra food is always left in the tank for him, and the few times ive had to move his hut to clean tank and seen him, his belly looks just as wide as the others, so I'm assuming he's eating enough...I'm thinking about putting him in a separate tank just for feeding to be sure, but i fear this will make him even more skittish of me. Also, I'm trying to plan for the next tank for them, as a standard aquarium isn't ideal. Preferably something with front opening doors would be great. Any suggestions? If suggestions are for bought tanks/enclosures please specify where so i can research. Biggest concern is how to heat larger enclosures properly for the ackies since they need 130-150 as subadults into adulthood. I have been reading copious amounts on all around ackie care and figured I'd ask here too. Pretty long winded so I apologize but tried to get as much out to avoid further explaining. Thanks in advance!