No a penalty as such, but the rental will be re-priced at the 2-day rate which could be higher than the 3-day rate you rented under.

This is especially common if you've rented for 5+ days (which will give you a weekly rate) and then return it early which will be calculated as at a daily rate, or if your original rate was a weekend rate that you no longer meet the conditions for.

Presuming this is a rental you've already started, call the number on your contract and they will be able to tell you what the new rate will be without actually changing your return date.