There are some true freshwater pipefish. By default, keep the thing in neutral, moderately hard water until you've established what the species is. I'd even suggest raising the SG to 1.002-1.003; this is unlikely to harm freshwater species, but will help brackish ones.
Examples of true freshwater species traded:
Microphis deocata
Dorichthys boaja (Wildwoods has these in stock)
Examples of brackish water species traded:
Microphis brachyurus aculeatus (usually as "Microphis smithi")
Enneacampus ansorgii
Use Fishbase to try and narrow down your fish, if you can't obtain a Latin name. That said, the four species listed above seem to be the ones usually sold as freshwater fish (as far as I can tell).
You should also try to determine the sex once you figure out what species they are as males can sometimes be agressive towards each other and as you have three, this is a possibility you may want to keep an eye out for. Either way though good luck!