Nintendo’s first major revision of the 3DS released to Japan in October of 2014 and to North America in September of 2015. The 2015 3DS introduces improved 3D capabilities, a C-Stick, new internal amiibo support,
and a new set of trigger buttons.
To my knowledge when buttons stop working properly, it’s either because the place where they are is dirty or they are just toasted as you said. Try cleaning it with some canned air and see what happens if not, take it for a repair at that point.