Here's the thing about minimums - there is a range of minimums depending on who you talk to. 3 is the very smallest of the minimums - the lowest of the low. I personally do not believe 3 fish constitutes a school any more than 2 fish does. The most common minimum number thrown out there for schools is 6. My ideal minimum is 9. It is my belief that if your tank is not large enough to accommodate a decent school of a particular fish then you should choose a smaller species.
And that leads me to my next point - emerald Corys are not Corys, but brochiis - a cousin. A much larger cousin. There are any number of Cory species that will do very well living out their lives in a 20 long. There is one that will not - The species you have. With good care, those fish will grow quite large - too large for your tank. Soooo, that leads to a crossroad - if you see a 4 foot tank (55 gallon) in your future, then I agree get more emerald Corys. If you don't, I would either swap them out for another species, or just leave it be, not getting more, and deal with that later.
125 - BGK, chanchito cichlid, pictus cats, silver dollars, palmas bichir
125 - cichlids (severums, bolivian rams, chocolate), rainbows ( turquoise, red), loaches (angelicus, zebra, kuhli and horseface), plecos (BN, RL and clown), denison barbs, tiretrack eel, pearl gouramis, betta
90 - Congo tetras, african knife, upside down cats, spotted ctenopoma, kribensis, delhezzi bichir
2.5 - betta