Thanks for the update! I'm glad to hear that you guys sat down and discussed your options and came up with the best solution for your family and the animals you keep. . . hopefully one day you'll be in a better position to bring a few lovely little froggies home!
Gizzy's been pretty busy lately, so I'll try to answer as best I can in the hope that it helps!
I do agree with Bek - in my opinion, two would be better than three in a 5g, and I would *personally* keep it to one. That's just my way, though.
These little guys will take full advantage of all the swimming space they can get - I've learned this by having them in larger setups (29 and 55g tanks), and water quality can be tougher to maintain in a smaller tank, because of the high-protein foods these frogs require. If you do choose to keep them in a 5g, you'll probably want to do extra water changes, and once the frogs reach adulthood, limit feedings to every other or every third day to keep the waste lower, and the water clean - my current 3 have been on an every-third-day feeding routine for over a year now, and are doing brilliantly (I did feed them daily until they reached their full growth).
I've had my babies in a number of setups now, and though they have done well in all of them, I felt that my frog-only species tank was ideal. That was 3 frogs in a well-planted 10g, with a Nerite buddy. I shut down that tank and put them into a community to make my life easier, but when things settle down again, I'd love to set that tank back up.
I typically keep my tanks with a sand bed, but am currently keeping my frogs in a tank with a 'river gravel' substrate. For a short period of time in the beginning, I had one frog in a tank with standard gravel. I have never had a frog have any issues with ingesting the gravel, but I won't say it isn't possible. If their food is on the substrate, they tend to hunt by pouncing - bringing up mouthfuls of whatever is down there - so I can definitely see accidental ingestion it as a possibility, though I'm not sure how common it is. . .these little guys are pretty smart. I have trained my frogs to eat from my fingertips or a turkey baster - this way they come UP for food, and don't pounce around on the substrate. You can also train them to feed from a dish, which would help keep them safe.
Hope this helps!