Yes, it could be stress or nerves. It could also be any number of other things, like food or drink that contains bladder irritants (caffiene, alcohol, etc.), some medications, fatigue (causing very sound sleeping), or large amounts of fluids. If you can recall some food, drink, or activity common to all three incidents, that might provide some clue as to likely causes. Since it has happened three times, it's worth discussing with his doctor just in case there is a physical cause. Bedwetting once or twice a year really doesn't justify using diapers, alarms, medications, or any of the other things commonly used to get kids to quit. I would put a plastic cover on the mattress and then something absorbent, like a large towel folded in half, under the sheet where he sleeps.