Three-week-old fuel is brand new. I don't even fill up my car that often.

No issue here.

I'm guessing those tanks have been sealed from when you filled them?

Gas goes bad primarily from absorbing water from the air, oxidation, and evaporation. Since the tanks were kept closed, the gas will last for years.

Gas cans that are used frequently need stabilizer because every time you pour a little to fill a lawnmower, fresh air gets in to replace the volume poured out.

What everybody has said here. I've got cans that have gas from years ago between the boat, weedeater, lawn mower motor cycle etc. I keep a little shy of 20 gallons on hand and cycle through it first in first out. That run just fine in anything I put it in. The 2 cycle mixed gas is the worst offender as I use so little of it it might take me years to make it through a gallon.

That said three weeks is nothing. The gas I put in the chainsaw two weeks ago had been in the can since the last administration, and it cranked up and ran without fail for hours, the only time it quit was when it ran out.

If you are concerned about it, you can mix 5 gallons at a time in your car with the remainder being fresh gas. I.e. if you have a 15 gallon tank, use 10 gallons of new gas with 5 of old.

3 weeks isn't all that long though, its probably fine as long as you didn't leave the cap off.

What kind of car is it potentially going in?

Definitely fine. I fill up my car like...once a month, if that.

Start worrying about it when it's last years gas, you're fine

Nah you're good

ChrisFix said once (in the first Mustang video at 7:40) that you try old gasoline in your lawnmower. If the lawnmower works, you can probably use it in a car.

3 weeks is fine. Add stabilizer now. It will be fine in a vehicle.

I just ran my snow blower on fuel bought in October... If I don't use it all, it goes in the truck in March/April. It may have done 89 this season... But I've never had issues with 87. Except for 2cycles... Buy the canned/ethanol-free stuff or switch to electric.

It will be fine. If you're worried about it, dilute it with fresh gasoline.

I once drained gas out of a car that had been sitting a few years. (I was fixing the car) I added it a couple of gallons at a time to my car's gas tank and never had an issue.

I’ve used three-week-old fuel and I was fine, but I’m not sure about pushing it past that. If you’re a car too, you could possibly drink it to get your money’s worth. If you’d rather be safe than sorry and don’t want to risk causing harm to your engine, though, I’d just take the loss and dispose of it.

Should be fine on more modern cars (ie. Everything with fuel injection, newer than 1990)

I run gas that's probably 1-2y old in my pickup and never had any issues.

Just beware that ethanol might hurt rubber lines if you have them. So best to mix it with some new gas over time