According to most of the dp sites, 2-3 per puffer is the smallest you should go. That being said, if you have the money, I'd go for atleast a 5-gallon if not a 10-gal, as they are very curious fish and like to have lots of room to swim and poke around in. Plus, with a 10-gal you could try a pair or a trio (1m, 2fm). If money/space/etc prevents that, I'd say try the 3-gal. If you can get one with a filter all the better, as they are pretty sensitive to ammonia and having to change the water out everyday (yeah, every day! they are messy!) will be pretty stressful on them too. I'd get the biggest tank you can, and get a good filter for it. They are not very hard fish to keep once they are settled in well, but they are a little picky.
kimmers318, what other fish are in with your puffer? I've been very interested in seeing what other fish work with the DPs.
Try the #### site.. lots of good info on it! Also, there are a few people on this board that are pretty knowledgable as well. Good luck, and let us know what you go with... DPs are great little fish!