it may not always be changed, it may be a constituent in a formulation such as a filler powder in a tablet, so it's a good question. But mostly if it's a pharmaceutical product we are only worried about the API itself, and how rapidly it degrades. But the expiration date of a product such as this has to be validated with data. 3 years is actually a long time so the product would be pretty stable.
Also, a raw material API would be less stable on its own, normally in a formulation, you would add other ingredients to extend its shelf life.