I agree, 3 females is most likely what you have there.
I would also agree that plants will probably get dug up. Once convicts hit maturity they like to move the substrate as they see fit.
You shouldn't run into too much aggression problems with 3 females, once the smaller one gets to be a close to the same size as the other two. With that being said if one of them decides to drop a clucth of eggs, she may get testy for a bit.
If you want a male, go to the LFS and look for one that is the closer to the size of the larger two females you have, but has NO orange on its side. It may also look a little "taller" than the others. Look for its top and bottom fins to extend to the same length or past its tail fin, it will most likely be a male.
A bit of warning, a pair of convicts may lay claim over the entire tank. The pair in my 55 gallon push the smaller female tankmate all the way into the corner. Convicts are almost as famous for being agressive while spawning as they are for being prolific breeders.
hope this helps.