I've not had much luck with those water heaters, if you read the specs some are only good to 15 degrees. A heated dog water bowl is a better option. I've never had mine freeze completely.
Three chickens in a coop might be okay depending upon how big the tractor house is. If they're snug and can heat each other, they should be okay. If there's more room than, say 12 square feet, you might try to protect them. Buffs and RRs are pretty cold tolerant, but have their limits.
I've had success by boiling water, pouring it into a one gallon plastic container, water jug is fine, and burying it in the sawdust or shavings up to the cap. It actually has stayed liquid at 15 below here in Colorado throughout the night. Water loses its heat more slowly, so the bigger the container, like two gallon maybe, the better.
Good luck, stay warm, Mary