Fin rot AND ich in tetra tank?

I have three neon tetras. One has a terrible case of ich and fin rot (his tail is completely gone) and seems to have gotten all this within 24 hours. The other two look fine besides being extremely pale/brown.. they're also a bit sluggish. The really sick one is a nice dark colour though... i put the sick one in one cup and the slightly sickish ones in another cup. I rinsed out the tank and gravel with scalding water and replaced it with a mixture of treated tap, bottled water, and stable water from my larger healthy fish tank. i cleaned the plants too. What i want to know is, can i move the two healthier ones back to the tank as long as i change the water daily for a few days and maintain medicated treatment while i let the other one stay in its cup until it shows signs of improvement? Or are they all doomed.