I have a mosfet h bridge design which works with a passive load of 3 amps @300vdc.

therefore p=v x i = 300 x 3 = 900 watts

so the whole circuits power disspation = 900 watts

how do i work out the power going through the mosfets?

is it 900 watts or is it P=I² x Rdson = 3² x 0.27 = 2.43 watts

which means that 2.43 watts is going through the mosfets and the rest in the passive load??