Power Losses in a 3 phase rectifier
simulate this circuit &ndash; Schematic created using CircuitLab

I ran a simulation. 



The simulation implies that no two diodes in the same block (indicated by the dashed boxes) conduct at the same time. Instead they take turns conducting the full DC current in an almost 50/50 raster. This would further imply that at any time a block would not dissipate more than the losses of a single diode. 

In this way each block behaves as though it were dissipating 224 W continuously.