Budget SLC is a franchise run by the group that also runs Budget locations in MCI, ATL, Birmingham...
What exactly does the rental agreement say about refueling locations? Does it actually state 3 miles? I've seen 5-10 miles, never 3 miles. I am not sure there IS a gas station within 3 miles of SLC. Even still, they should not have charged you for a full tank in any case. Perhaps one gallon or something.
Every rental I have made with this particular Budget licensee in the above 3 locations, I never receive my frequent travel points for whatever partner I include on the reservation. Yet, they always manage to add the "fee" for it to my contract.
This licensee is on its own system, it is not on the corporate Avis/Budget system. They basically connect to Avis/Budget's corporate system to retrieve your reservation then manually enter all of your reservation data into their system.
Their contracts read quite a bit differently from a corporate location. Some interesting reading. Not all bad, by any means... just, different.