I returned a vacuum to Costco after 3 years because we had new floors installed
That's a surprising move. Why did you make this decision?
We took out the carpets and put in hardwood floors, so I didn't need the vacuum anymore.
It's okay to return goods to a store for a refund if they haven't been used, or if they are defective in some way. However, if you have already been using this vacuum for three years, then you have gotten your money's worth, and returning it is essentially a form of theft.
They will never know it's 3 years old. I barely used the thing.
I doesn't matter how much you used the vacuum or what shape it is in, the point is that the vacuum has passed it's return date and is now a used item so returning it for a refund is dishonest. It's okay to take back new stuff, but don't do it when an item has been used.