More likely than not they WILL charge for returning the car early. If OP booked for a weekly rate and then returns 3 days early they are changing the terms of the contract and agreement. IF OP has a rate quote for a week and only needs for 4 days (example) the rental company can and WILL charge for 4 days rather than the week. Think of it...if they didn't charge for returning 3 days early or even 2 or 1 day early everyone would do a weekly rental...get a low rate and return early. Have you rented a car before? Have you returned one early before (not by hours) BUT THREE DAYS and NOT had them charge you.
The rental rate you get is based on the information you provide to the rental company. If you provide incorrect information they can and most likely will charge you a different rate. This goes for early returns, not being able to provide eligibility on a discounted rate, etc. I was once asked for my Costco card by Alamo.
OP you will end up paying more than your original quote. You might call the company and see if you can get a better rate, but if your original quote was based on a week and you need less than a week expect to pay more. Even if you don't let them know you are returning early and just "drop it off" early they will and can charge you more. You might rebook a 2nd reservation and then call and discuss modifying but be prepared to pay more as the information provided in the above responses is incorrect from all my experiences and from reading information on this subject on the transportation area here, fatwallet travel area and flyertalk.
POST ON THE TRANSPORTATION AREA TOO AND SEE WHAT RESPONSES YOU GET THERE...LOTS OF RENTAL CAR EXPERTS THERE!!! Don't rely on I've rented and it shouldn't be a problem. You are totally changing your reservation and terms so expect changes in the price.