Generally speaking, a larger group is best, say five minimum. However, these should always be acquired at the same time. The three on their own for two months may well have formed their hierarchy, and intruders may meet with trouble. The gender of the three would impaqct on this too. The interactions of the present three should suggest whether things are OK or not. Loaches are very social fish.
Keep in mind they get large, eight inches minimum and up to a foot or more, depending upon environmental conditions.
Byron.