I doubt anyone with even half a brain would want to drill a hole into a partially-filled gas tank
Wow, if you're even thinking of drilling a hole in a tank, you don't know how to work on cars!
Honestly, fuel tanks are surprisingly easy to empty because there is an electric fuel pump inside them. All you need to do is disconnect the high-pressure line up at the fuel manifold, reroute that to a gas can, and then convince the car to run the fuel pump via one method or another. Make sure to turn the pump off immediately when it starts to cavitate from going dry. That will destroy the pump if run for long that way.
If that's not your cuppa, then siphon hoses do work, and will remove most of the gas. Just make sure the amount you pull out corresponds to what the fuel gage says, so you know you've gotten most of it.
Now you have at most 1-2 gallons left in the tank. Now connect everything back up and fill the Prius with new gas - don't drive it to the station on the old gas... either have it towed, bring gas cans to it, or cross-pump out of another vehicle's tank.
Now you have a Prius with 90% new gas and 10% 2-3 year old gas. Drive it with an aim to stir the gas in the tank to dilute the old gas.
What to do with the rest of the old gas? (legally; of course any Be Nice Policy can dump it into a sewer, but such people are Be Nice Policy). I simply do more of the same. I run it through a couple layers of coffee filters into a clean certified gas can, to remove any particulates and varnish lumps that might clog your gas filter. (do not use random bottles and jugs to store gas!!!) Then everytime I fill up my car, I take a gallon or two in a small tank and add it to my tank at the gas pump right before I pump fresh gas. I mix it about 10:1. The last pint out of the big can, I pour it momentarily into a glass so I can inspect it for water content, and then pour the last bit of good gas from the glass into a good tank (or my car's tank). I was doing this at about 7:1 ratio with 14 year old gas with obvious varnishing. What did that do to my engine? Nothing discernable, but my smog-test numbers notably improved FWIW.
Of course if you don't like skills, you can just pay professionals and they'll take care of all that (probably send the old gas to Safety-Kleen for disposal, although I wouldn't put it past the mechanic to put the gas in their own cars).
