Have a look around the Dollar/Thrifty forums to see an important difference in their business model for one-ways:
Hertz, Avis/Budget, and National/Alamo cater to daily rentals. So for one-ways, their daily rates are higher than roundtrip, with no separate taxable "drop fee." Ex, roundtrip $45/day, one day one-way $85/day, 2 days one-way $80/day, etc. As the number of days increases, the predicted daily mileage decreases, so the daily rate adjusts accordingly. This works best for long distance, short time rentals, and daily business travelers.
Dollar/Thrifty has a flat rate, separate drop charge regardless of days rented, added to the same low daily price as roundtrip. Ex, one day at $30/day, $500 drop charge, or 7 days at $30/day, same $500 drop charge. As the number of days increases, but the drop charge remains the same, pricing approaches and can even be lower than the other brands. This works best for short distance, long time rentals, and some leisure vacationers. The result is fewer transactions, which allows Dollar/Thrifty to have a smaller fleet, with fewer employees, but more revenue per transaction.
If you're doing 3 days, the first example is probably a better choice for you.