The size of the fish and size of the bag restrict whether you can transport more than one fish per bag. Just put enough water in the bag to cover your fish by a couple of inches, then "grab" air to fill up the rest of the space, and rubberband the end. I'd then set the bag in a cooler to transport. Keeps the temperature more steady, plus catches any leaks.

fish are often shipped across the country in bags and insulated boxes, so I think a 3 hour journey in a car is ok if you keep the bag cool or at the temperature the fish normally survive in. However, if you enjoy fishing you may find having a live bait box a great plus. check the link

Here is a link that might be useful: live bait box

How big are the fish? The suggestion by maryhm is accurate. You might consider adding a little ice. You don't want to freeze them but if you cool them off and keep them in a cooler then their metabolism will decrease and they will need less oxygen.

And a good way to keep the oxygen levels high is to put in a teaspoon of hydrogen peroxide in the water at a rate of 1 teaspoonful per gallon of water per 3 days.
When we moved from NYC to Indiana the kids would not leave their fish behind so we put the darlings in a five gallon bucket with a bubbler and a dose of the peroxide. At night when we stopped at a motel the first thing to be done was to plug in the bubbler. All fishies survived, no thanks to the changes in temperature. I suspect the AC helped there. The fishies were in the bucket almost two weeks. Sandy

Wow, Sandy. I'm going to use your hydrogen peroxide method, too. Cheaper than the live bait box (though that's pretty cool).
Vanessa