In a 20-gallon tank, you're much, much better off with a single pair than any other option. If you get a trio, two will pair off and harass the single female and probably kill her. Two males in a space that small, and the males will fight a lot (the females less so) and you'll end up with one dominant pair and one weak, frightened, unhealthy pair that never gets to eat properly and will probably eventually die from stress and malnutrition, if not from infections.