Of course, you don't know how old they were when you got them.
There are also factors that shorten the ideal lifespan of fish in aquaria. Stress, physical, chemical or social will do so. Fish kept in large tanks in stress free environments will live longer then cramped quatered poorly maintained inappropriately stocked tanks.
You must be doing something right to have kept the entire group for three years, but I would be a little suspicious of them turning over in short order. Have you added/removed/changed anything recently - meaning not just physical things, a change in water change regime, feeding etc. ?