Do you mean Micropoecilia parae melanozona? These have been doing the rounds in the UK shops recently.
These tank-bred fish aren't particularly difficult to keep, but like other Micropoecilia sometimes they seem to fail for no obvious reason, though environmental stress, diet, and perhaps opportunistic bacteria could be to blame. Avoid extremes of temperature, and certainly keep them in alkaline, and preferably very slightly brackish, water conditions, around SG 1.002-1.003. Yes, they do occur in freshwater as well as brackish water habitats, but the use of brackish conditions does seem to have mild tonic effect on Micropoecilia species.
If you only have a trio, get them breeding in slightly brackish conditions, and once you have some fry to play with, you can divide them into ones you're keeping in freshwater and once you're keeping in brackish water. If both sets do fine, then there's nothing lost. If you keep your original trio in freshwater and they don't succeed, you can end up losing everything!
Cheers, Neale