A trio would be fine in that size, just have simple aeration and do not use power filter as it dursturbes the surfave to much, a sponge filter would be fine.
Which species are u getter?
A friend keep's wrestling HB's in a large drain pipe in hid fish room, it's coveres and used as a shelf some of the time, but he rear's his fry in that, currently has over 30 fry at 1-2" in there it 3-4 " deep and runs for about 2-2.5 meters.