12 x 3 = 36amps. It only has a 30 amp rated breaker. Your over the limit at 3 warmers. If it trips with 2 warmers, turn one on and let it get up to temperature before turning the other one on. The inrush current of the cold warmers is what is causing the breaker to trip.
I have a 58 amp gas generator that need to provide 7000kw but it has a 30 amp breaker. The braker will go off with 2 or 3 12 amp warmer how can I make it good for 4 warmers