I keep all of my adults in 32-quart tubs in my rack system. I keep several 48-quart rack systems around and that's what I use for breeding and egg-laying. After they've layed their clutch, they go back into the 32-quarts.
As for a thermostat, I would not recommend running 3 racks off of one thermostat just because of the fluctuations in heat tape. Also what happens if the heat tape burns out or the power comes unplugged in the rack that you are probing. In that case, the thermostat is going to try and crank up the heat and will toast the animals in the other two racks. My suggestion would be a 4-zone GCS thermostat. It's a proportional thermostat that controls 4 zones independently with 4 separate probes. They also include built-in night drop and an audible alarm. For the money, they're the best way to go (it's cheaper than buying just two Helix units). You can contact me if you have other questions about them.
If you have any questions on either one, let me know. I manufacture racks and breed balls 
Take care,
Jason @ Jason's Jungle