Five of them would be a lot for a 10-gallon as they get older; even though they do stay small, they are an active species and that would be a crowded enclosure. Three would be just fine there, though! Extra heat is not required as long as your home is warm enough (mid-70s) but they will appreciate a heat mat as for reptiles and may do better with it.
Es chirp a lot, too, which is very interesting and cute unless they're doing it in response to harassment. You'll have plenty of social behavior to watch during the first week or two especially.
Personally I'd give them a 40 - 50 gallon, as with my limited experience with hermits, (mainly watching wild ones) they are nomadic in nature but still have a "base of operations" if you'd call it. All I can really give, true crabs are more of my forte.
And I've had hermits for ages so 6 inch of sub is fine but the 10gal would only do for 3 hermits, you also need a heat mat but it need to be on the side of the tank not under it(I use a heat wire as it's easier and you can put it inside the tank