Lock-step execution made sense back in the old days, when instruction cycles were predictable and there were many chips making up a system.
It's far easier to implement on the same die, but it make far less technical sense -- it's not protecting against likely faults. The primary value seems to be in marketing.
As for three processors and voting... failures are going to be vanishingly rare. If they weren't, far more computations would be silently corrupted.