Glad to see everyone gave awesome advice, but I'd like to mention one thing, AZriverrat you're information is wrong. When a goldfish becomes stunted, yes their growth physically stops, but so do their organs. It is a complete myth that the organs continue to grow.
As for your situation Dara, I can completely relate.
I have a single tailed goldfish on her own, she was originally in a 5 gallon tank (with Star who we sadly lost) and about a year ago I upgraded her to a 3 foot/23 gallon tank. Her growth is very slow and so I'm keeping her for now until she is big enough, then I will be re-homing her to a pond or organising a larger tank.
Ultimately single tailed goldfish need to end up in a pond or a really large tank (at least 6 foot in length) but I can appreciate this isn't possible for everyone -myself included, so I've been considering re-homing and will find my little goldfish a pond when she's big enough. Or if her growth is really bad, I will look into building or purchasing a larger home for her.
You need to think what's best for you as well as the fish, may I ask how long it will be until you move? Simply because the longer those fish are in that small tank the worse it will be, Lucky was only in her small tank for 5 or 6 months and her growth has been massively affected from what we might think is a relatively short time.
Have you considered getting a large plastic storage container in the mean time? I have seen plenty of people use them as temporary accommodation, as long as the item in question is food safe and can handle holding the water, it would be fine
Can I also ask how the goldfish behaviour is with the remaining goldfish? You will want to keep up maintenance as the tank is more than likely cycling, so I would suggest doing large water changes weekly, take out as much water as you can (around 80% should do it) and always use conditioner for water that you're putting back in. This will at least keep the water quality good, until you can get something bigger.
It would also be worth getting a pet store or buying a kit to test your water quality, you'll want to keep an eye on it, because if one of the bad elements i.e. ammonia, nitirite or nitrate, spike you will need to know about it and be able to re act accordingly -generally just 50% water changes daily until the value is back to 0ppm or in the case of Nitrates under 40ppm.
If you have any questions feel free to ask, I hope what I've written makes sense!