How about this 1? What changes are required?
For my first heater & thermostat RELAY control schematic drawing in Post #23, no changes are required. You can simplify the logic by only using 1 contact for the Safety Shutdown thermostat to cut off all 3 heaters (as I did in the LogixPro simulator in the attached pictures).
Sometimes, relay logic requires proving that it works to the non-believers, so here are a series of pictures using my original logic, using a LogixPro PLC simulator program with a subroutine that simulates changing temperature from 80 degrees F down to 45 degrees F, then back up to 80 degrees, repeating continuously. I can post the LogixPro program that generated these pictures, if anyone is interested.
In the first picture (Temp = 79), you can see that the safety shutdown had kicked in and cut OFF power to all heaters.
In the second picture (Temp = 65), you can see that one heater output is ON, as required.
In the third picture (Temp = 59) you can see that 2 heaters are ON, as required.
In the fourth picture (Temp = 53) you can see that 3 heaters are ON, as required.