There will be no sound in common among the 3 listeners. Consider that sound travels at a relatively fixed speed over distances regularly found in concert venues, so the sound will take longer to reach person A than person B or C They will therefore hear anything that happens on the stage later than B or C.
But the PRACTICAL difference will probably be negligible. For example, a light show will be "close enough" for all three viewers if the distances are similar to those found in an amphitheater or indoor venue. You'll begin to notice the distances in a setting like a sports arena - try to synchronize two sides of a college football stadium in a responsive chant, and you'll see the lag effect. Neither side hears anything in common at the same time - but both sides hear the same sound at different times. Or have everyone clap together; that's pretty much a great way to guarantee a train wreck.
For sonic pressure (volume), you won't be able to compute it in a meaningful way without knowing the distances, at least relatively, from the sound source.
Even light will not be exactly common to A, B, and C, but only in a very abstract kind of way. They will all experience the same show together, albeit at slightly different times.