"Yes, I remember the 727 which was build that way. Upper and lower rudder.
What, if upper says left - but lower says right?"
Errm, you turn the faulty one off?
To make the point to a pilot who had failed to notice a PFCU red warning light (in the VC10 simulator) below V1 (mandatory abort for that type) - we gave the crew a middle rudder PFCU failure and a lower rudder hardover. Stupid sods then spent the next 10 minutes fannying about trying to get the failed middle rudder back instead of first attempting to turn off the 'hardover' lower rudder (electrohydraulic servo - the fault was an input signal error)....
But they worked it out eventually - and the pilot in question never missed a 'below V1 red light' again!
But if all 3 PFCUs are driving the same surface, I can well imagine it breaking up if 2 say one way and the 3rd says the other. Unless there's some other protection system, of course...
Is there?
And the AA accident was down to inappropriate and ultimately fatal mishandling. All a/c have their quirks - you don't leave flap down above Mach 0.9 in a Hunter, you don't select airbrakes with flap in a VC10 - and you don't pedal the rudder of an A300/310 like a demented harmonium player!