Keeping a trio of seahorses will often work out very well. In most cases, it’s not a requirement to keep seahorses as mated pairs. You needn’t worry that the odd man out in your threesome is going to be too lonely. Captive-bred-and-raised seahorses are normally very gregarious, highly social animals that very much enjoy the company of others of their kind, and although one of the seahorses won’t have a mate, there will always be another pair of seahorses within sight in a small aquarium such as yours.
As long as there is no aggression or hostility toward the third wheel — and there usually is not in situations such as yours — then there’s no need to try to even up your herd of ponies. In most cases, a trio of seahorses will get along splendidly, and they may all participate actively in courtship and breeding at some point. That is to say, the same two seahorses may not be the ones that breed exclusively each time a brood of babies is produced, and the ponies may change partners once or more over the ensuing months and years. (If given a choice of partners, domesticated seahorses tend to be rather promiscuous in the aquarium and will often breed year-round in captivity if conditions are to their liking once they begin to feel at home.) Within a species, the coloration usually doesn’t matter, and I would expect that your black Brazilian seahorse (Hippocampus reidi) to intermingle freely with other H. reidi that are differently colored (e.g., yellow or orange) and become good buddies with them if you want to try some more colorful Brazilians this time around.
Best of luck with your H. reidi and all of your fishes, JetMech!