If you split it in 3, it shouldn't be too large. Might be a bit on the tall side but the best thing you can do for them is separate them. They are not social animals and do not benefit from being housed together. They would most likely prefer to be apart.
If they are separated, there is no reason to feed them in another enclosure. If you want, you could use that 20g for one of the snakes.
As far as heating, how are you measuring the temps (what kind of thermometers)? What kind of heating devices are you using? How are you controlling them? (thermostat? rheostat?).