Pair of physicists bust a 350-year-old conundrum in a report that proposes a transfer of energy through a sound pulse causes clocks to synchronise
I didn't even know this was a thing, and now I know the solution. Fascinating!
Mythbusters did a similar thing with metronomes: you can see part of the episode here.
If the clocks are hanging on the same wall it is possible that the structurally transmitted vibrations would be more effective than acoustic waves.
I think it will follow the logic of most other binary things - it will alternate. What I mean is this: assume that during a single swing (no return), a pendulum going left-to-right is 1 and a pendulum going right-to-left is -1. With two, the sequence is {1, -1}. Three, it is {1, -1, 1}. And so on. They would always go opposite of their immediate neighbours, assuming that they are closer to some than others - with three in an equilateral triangle, I would say either they would sync at a 120 phase, or completely wreck their inner sync and eventually cause them to stop.