For a binary signal, it's impossible for all three to mismatch. For a more analog signal, all three will generally mismatch to some extent whether it's in time

For a binary signal, it's impossible for all three to mismatch. For a more analog signal, all three will generally mismatch to some extent whether it's in time or in space or both. Fault tolerance and fault detection are two separate but often coupled concepts. Systems can be designed to be inherently tolerant to a fault without detecting the fault (and good designs often are). All faults need to be detected eventually, though, so that they can be repaired before more faults occur and compound into a broken system. There's typically very tight timing requirements for fault tolerance, and significantly looser timing requirements for detection. As a result, you can often solve them differently. In a 3 string system with voting, it's often the case that the median signal is used for control without any interpretation of "goodness". That strategy works fine for short time periods of fault tolerance, as two strings would have to produce bad signals for the system to be affected. Separate from the median voting control path, you would then have a variety of consistency checking algorithms looking at the three signals and trying to intelligently determine whether any of the strings have failed. Those algorithms are often stateful and complicated, and rely on heavy filtering to avoid false positives. When a fault is detected, at minimum it needs to be communicated to an operator. In some cases, the detected fault will also trigger a "fault response", i.e. disabling the offending computer. In this case, it sounds like maybe a fault detection algorithm had a false positive that disabled the computer, and the same algorithm was running on all three computers. Despite there being 3 computers, it doesn't sound like this is a 3 string voting system. Rather, each of the three computers are independently able to control the system. The 3 strings exist for redundancy rather than for fault tolerance. Fault tolerance is provided by having two computers that cross-check everything they do, and third computer is there so that the cross-checking is fault tolerant. Two string redundancy is very common in automotive and aerospace.