The smallest tanks I have are two 5 gallons with no fish in them, but my smallest with fish are
10g with 2 dwarf puffers
10g with a trio of Gardneri killifish and a 1.5" asian bumblebee catfish (QT)
10g with female Mogurnda clivicola, preparing her for breeding (Good food)
Genesis - by "sucking loach," you don't mean Chinese Algae Eater, right? If so... please do not put it in a 3gal tank. They need more like 20g to survive to adulthood, though they could probably survive for a bit in a 10g until they ~4 inches long or so.
My smallest tanks in use are 5gal. I do not keep my bettas in anything smaller unless they are extremely ill and need to be kept confined and in shallow water to recover.
Lol, even my Betta lives a 10 gallon. I have several small aquariums for fry, they are 1.5 I believe, but they are all empty at the moment. . My smallest with water in it is 5 gallon, but it's empty and the filter wont run.