A dwarf puffer would be ok I think. I kept one by itself in a three gallon hex for several years in high school, and I was ignorant about all kinds of things back then (like weekly water changes). You could feed it the tiny snails that most lfs would give you for free.
I also have to say, there is no reason with good filtration, sufficient hiding places and regular water changes you could not keep a trio of guppies in a three-gallon tank. It would be tight yes, but not impossible, and far from cruel.
Along these lines, I know there are a lot of shall I say "overstocking crusaders" on this forum. For the most part the warnings are warranted. I would however like to remind everyone of the boy who cried wolf. If you call overstocking on virtually every tank, people will stop taking your warnings seriously. Finally language like: "you can't”, "period!” and "never" should be replaced with words like: "inadvisable”, "I wouldn't”, and "you might want to reconsider".
Please, nobody take this advice personally, it is merely an observation coupled with a suggestion.
