The data I quickly looked at for the decomposition of the main component (sodium chlorite) gives a thermal decomposition temperature of 190 Celsius. That's not to say it does not happen below this (it's a statistics thing) but I can't imagine it would happen to any significant amount at normal room temperature. In other words, I think the tablets should be stable/undiminished for a long time. Three years isn't long.
Moisture is the main threat really, assuming the tablets aren't being kept on the dash of a car in the summer sun. If the foil is punctured then all bets are off with regards to how long the tablet remains effective.
I have Portable Aqua Chlorine Dioxide tablets secreted in various crannies and nooks in my kit. That has a stated
four year shelf life and most of my tablets are probably around that now. They show no signs of any damage though and given the above, I would still use them; probably for several more years without worrying about it.
That said, if money is no object there's certainly no harm in swapping out old for new, if only for peace of mind. My money is, unfortunately, an object.
(I only keep the tablets for 'emergency' use as I normally carry a Katadyn pocket filter, now to be replaced by my new Sawyer
)