I would treat them, if i have any fish that shows up with a sickness, i remove that fish to isolation, but then still treat the whole tank. If the fish caught it in there or allready had it while he/she was in there, that virus/bacteria could still be present. Especially with ICH which only seems to hit really stressed out fish, the other two might not be as stressed so they are fighting it, but they could still have it.
