-- The net effective resistance of three 1200-ohm resistors in
parallel is (1200/3) = 400 ohms. That's what the battery sees.
-- Power = (voltage)² / (resistance)
= (12²) / (400)
= 144 / 400
= 0.36 watt .
There's no such thing as "power in the circuit".
0.36 watt is the power dissipated by the resistors.
It's the rate at which the battery must supply energy,
and the rate at which the resistors blow it off in the form
of heat, for as long as the battery lasts.
