I do not know the difference between your qt 10 gallon tank and your regular 10 gallon tank - but I am hoping the qt tank has a good filter running on it too and it, too, is cycled.
If this is the case, splitting your fish into different tanks is the best thing possible. Put one 2 incher in by himself, and the other larger one with the smaller one. This will give the greatest amount of water volume per fish possible. As commons, these fish really need at least 20 gallons apiece. As small as they are, they may be able to survive in 10 gallons
apiece, but 3 in a 10 gallon tank simply will not work.
Any time a fish is stressed, the water is less than perfect, etc, opportunistic parasites and diseases will surface. Ich is just such a problem. Ich can lay dormant in the gravel in a tank for months to years, popping out and attacking any time a fish has problems. If the water gets too cold or too warm, the ammonia levels climb, the nitrate is high, the nitrites are not zero. Any stress can set off the opportunistic ich.\
To treat the ich that is attacking at this time, you will have an easier time if you attack it in a slightly different way.
The best thing you can do is to get a larger tub - as big as you can - at
least 20 gallons. Move all the fish to the tub with the filter(s). Treat the ich in the tub - treating ALL three fish. Even if they are not showing the tell-tale spots, ich can be infesting a fish - a fish can die of ich and never show the spots. The spots are the parasite under the skin of the fish. You can only kill the parasites when they are free swimming in the water.
So, move the fish to 100% clean, new water - at leats 20 gallons. Move the cycled filter(s) with them. Dump ALL the water and everything from the two 10 gallon tanks. Boil the gravel. Clean the tanks. Reset them all. Then, when the ich is controled in the bare bottomed tub, you can move one larger fish in one 10 gallon by itself and the two other fish into the other 10. This will give them the most water volume you can.
Look to getting a larger tank as soon as possible. Larger water volume, better filtration will make keeping these fish MUCH easier and will allow them to avoid stress problems with ich and other diseases.