The ability to use this particular muffin mix would depend on exactly what was in the muffin mix,what you needed to add to it, and if there are not bugs infesting the product. The reason for expiration dates is that some ingredients can either get rancid or become ineffective. If the muffin mix only needed water and perhaps an egg it probably contained some type of shortening and may have become rancid and therefore not advisable to eat. As far as bugs are concerned, there are several types that like flour/wheat etc. If you saw any web like structures in or on the box discard it in a sealed bag, take everything else out of your pantry and clean. Inspect other boxes of mix and discard any that have webs or other evidence of bug infestation. This can happen in the best of pantries.
If however you needed to add egg, water and oil it is probably eatable but you would need to add a bit of baking soda or baking powder because the product might not rise correctly. You would mix 1/2 to 1 tsp of baking soda into the dry mix before adding liquid ingredients.
Since you are talking about 2-3 yrs after the expiration date I would do two things before just eating the muffins. Smell the mix, if there is any "off" odor discard it and secondly after the muffins have been baked, take a small taste and again if it is at all "off" discard the product.
There is some leeway in product dates but you get a fresher
taste and better results if you are using them within date. I have made some mixes that are past date without any difficulty or health concerns. I would not use eggs, milk or other dairy products past their dates however.
PS better check the rest of your pantry to see if anything else is outdated