I have four Boesemani rainbowfish (one male and three females) in a heavily-planted 29 gallon at 7.0 pH, but I honestly feel that I am doing them a disservice by keeping them in such a small tank. You may want to consider purchasing a larger tank if at all possible -- then you could increase your school size as well. I'm planning on upgrading to a 55 gallon and adding at least two more rainbows.
Don't get me wrong, I am confident that you will be successful in keeping three rainbows in a 20 gallon long, but you will be able to observe more of their natural behavior in a larger tank with at least six specimens. Please understand that this is simply my opinion -- I am not trying to dissuade you in the slightest, I'm just a proponent of understocking.
You may also want to consider getting either three males or one male and two females. If you only have two males, the meeker of the two tends to get harassed relentlessly, especially in the morning hours when spawning normally occurs. I highly recommend purchasing directly from a breeder. The rainbows that I bought from my lfs were rife with disease and had to be treated in a hospital tank for Columnaris and secondary Aeromonas infections -- probably brought about by inbreeding, stresses of travel, and subsequent overcrowding once they arrived at the lfs. If you purchase your fish from a more reputable source, you will find them to be quite hardy (and absolutely voracious eaters, let me tell you!)
