Hi tophat, let me see if I can help you here. It IS possible to put one into a 2.5 gallon, but I don't suggest it. It's a difficult situation to maintain, and with needing a heater, it can be difficult to keep temp under control and find a heater of the right size. It's also quite a squeeze with waste levels when the fish gets a bit bigger. The size of filter that will even fit onto a 2.5 gallon makes for a somewhat crowded and unattractive aquarium. I've done it when I took in an orphan for a while, I had nowhere else to put it, so gave it one of my 2.5 gallon mini bow tanks until I could find it a home. I wouldn't do it again or for a permanent tank. Now, the scenario of 3 in a 10 gallon is good, even better would be 3 in a 15 gallon. I will say LOTS OF PLANTS for these guys, they're playful as much as they're aggressive, and they all have individual personalities that you'll soon fall in love with. If you don't want to try live plants, silk is the next best thing because they move around more like a real plant and they last longer. They're also weighted on the bottom so they don't float when moved or disturbed. Rock structure with the plants would make for a perfect home for the puffers.
If it were my tank, I'd cycle it with a few small feeder guppies so that when ready, the puffers will have something to chase around instead of each other. Regular water changes, foods other than the snails offered often without overfeeding, and lights turned out at night, and they should do wonderfully.
Let us know what you decide to do and if you need any other help.