When the Pittsburgh Symphony chose to forgo a traditional music director following the departure of longtime chief conductor Mariss Jansons, many in the music world wondered if the three-headed conducting team the PSO put in place could work. But three years in, the titular head of the triumvirate, Sir Andrew Davis, says it’s all working out fine. Still, he says, the arrangement does mean that it’s taken him a long time to get to know his orchestra…