from captain87
I think it would be the hardest abnormal situation to manage but it's nearly impossible to happen according the system's redundancy. A Triple Hyd Failure would mean that all the three systems have a leak and/or the engines are out with RAT locked in its IN position ... In this case I think that the Mechanical backup wouldn't be possible according the loss of the thrust levers efficiency that are hydralicaully operated.
I'm not sure ... but I believe this extreme situation should be out of consideration and I hope that the engineers have worked fine in order to avoid it.
captain87 (airline pilot student)