Regardless of the type of Goldfish
I'm afraid the best option here is to take them to the lfs to be rehomed. I know it's very sad as I have had to do this with fish before but it's for their own good really. A single Goldfish
needs about 30g to itself, the three of them together I would say should be in no less than 50g tank, but really should be more. The thing with Goldfish
is that they do actually get huge! Regardless of type they can grow to about 12"! IMHO they should be kept in ponds rather than tanks. They are also huge waste producers so make a terrible mess of the tank in a short while raising amonia levels which are toxic to the fish.
If I were you, I would return the Goldfish
to a lfs (most stores will take fish from you. They may not give you anything for them but they might give you some store credit or part exchange) and get 6-10 white cloud mountain minnows if you want to keep the tank cold water, or the same amount of neon tetras if you decide to put a little heater in there.
If you do return the Goldfish
either replace the fish right away with the minnows or tetras, or feed the tank a tiny pinch of food daily just to keep the tank cycled until you add the toher fish.