no, because it doesn't quite work like you think....each phase would do the powering out of step with each other, they don't all switch at the same time. in each cycle, each phase is taking a turn a 3rd of the time. so your PWM is really just working 2/3rds of the time.
You can imagine it like a 3 cylinder engine in a car, and cylinder one is misfiring because its not getting the pulse to the spark plug. What you are taking about with wiring the FETS together, is just like linking the "dead" spark plug to one of the plugs on the other cylinders - it would get it to spark, but at the wrong time, and the engine would still run wrong. Its the same with the dead phase, you could get it switching the FETs, but it would be at the wrong time, and cause unstable voltage (probably with a higher ripple).
The normal cycle would fire phase 1, phase 2, phase 3, back to phase 1 etc in that cycle.
By bridging you'd have no phase, phase 1 & 2, phase 3, no phase etc etc
so all you'd do is increase the pulse on the second part of the cycle, as it fired two phases at once, and it would still have the dead spot on the first phase of the cycle. It would probably confuse the PWM controller too, and that could cause things to go funky on the voltage output.