Calculating heat generated by resistance wire on 3 phase current
Power is voltage squared divided by resistance. That is the easy way to calculate the required resistance.

Calculating one leg is sufficient for 3-phase balance systems.

The easy and inexpensive way to regulate the heat is to turn the power on and off. That is all that is necessary in a lot of applications. If necessary, you can use solid state relays to turn power on and off cycle by cycle.