They don't seem to mind Mild salt (1/3 or less recommended amount).
What you can do to make their life easier is to provide them with tight hiding spots (wood, rocks and other tight to the ground things).
They will dig themselves into those spots by removing substrate, that being said, the substrate should be sand or other easily moved substance. they will bury themselves in sand if you provide it for them (but I wouldn't expect you to change the substrate for 3 fish)
Also, don't expect to see them too often, kuhli's are excellent hiders..
you also should suppliment with Shrimp pellets.
Is the community tank the 15Gal planted? if so, you should NOT be adding salt with plants.