I can't site the episode, but I vaguely remember an episode of TNG where Barclay and O'Brien are discussing the safety of the transporters, and they mention that there is redundancy built into the pattern buffers. Perhaps each of those dials correlates to one pair of pattern buffers, meaning one would work in a bind (and possibly make the teleport faster?), but all three is the safest (and possibly slowest) way, with two being a happy medium.
SA295
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