Of course, even with triple-redundant systems failures can still occur.
Air France 447 [1] three independent air data systems, two of them failed due to environmental conditions
XL Airways 888T [2] three independent AOA sensors, two failed because the plane was washed without the right covers in place
US Airways 1549 [3] two independent engines, both disabled by bird strike at the same time (No fatalities)
Qantas Flight 72 [4] three independent inertial reference units, bug in voting system if a single sensor's output had multiple spikes 1.2 seconds apart (no fatalities)
An in the data centre, no amount of power-supply redundancy will save you if a technician pulls out the power cables on the wrong server :)