1st, if you are trying to heat through wood with an unregulated UTH, it is never going to get through hot enough, or if it does then it's going to so hot that it's going to catch the wood on fire.
2nd, thermometer not thermostat. Thermostat is what you should have on your UTH, thermometer just tells you the temperature.
3rd, DON'T! You're just risking cannibalism of one for no gain except that you're cheap. Which is 50% of the source of your problems. The other half is that you don't listen to advice if it contradicts what you want to hear.
4th, If you can't afford to properly house what animals you have you shouldn't be getting more with the intent to make even more that you don't have housing for. You've had them together for a long time without them breeding, either the conditions are terrible for breeding (still possible) or they are the same sex. If you complete this project and it works then you now have a reasonably proper house for each one individually, not for three.