In general, if you are only dealing with flowers, the only thing you might lose over 3 years is potency, smell and color. Flowers are generally safe to consume with no expiration.
For food grade oils and edibles, it is strongly recommended to follow the expiration date due to the possibility of it actually going bad for you. The same is also true for high concentrations of terpenes. If you are storing large quantities of terpenes, it's recommended to not store them for more than 2 years.
MMJ usually has an expiration date on the packaging so you know how long it's good for. Again, if it's food grade, you probably want to follow the expiration date. If it's flower, don't worry about it.