As I recall, in normal circumstances, all 3 are engaged in cruise, but one is the master and the other two are slaves. The centre does not have any backdrive capability for the controls so can never be master, as someone else pointed out. I heard a rumour the right becomes the master the first time it is engaged after power up, and then it alternates each time it is re-engaged, but you'll never be able to tell in the flight deck.