Hey Gina, welcome to AC! :welcome:
It sounds like you have a few things going on that could be causing problems. First, that's an awful lot of fish in a 30 gallon tank. Especially pond goldfish! Second, you're adding one med after another and only doing partial water changes between!
You're going to want to keep the fish in a much larger tank, even if you are only keeping them there for a few months. If they are small enough now to physically FIT in a 30 gallon tank, they must be young goldfish. That makes it even more crucial to keep them in a large tank. Young goldfish can grow 2 inches or more in just one month! If the tank is too small, even for a short time, it can cause deformities and illness. I suspect that the stress of moving to a small tank may be what triggered the buoyancy issues in your fantail. Stress can cause minor problems to suddenly blow up into big ones.
The good news is that if the fantail is still eating and able to swim around, the swimbladder problem can be managed. You may want to first move all the fish into a larger tank. She can go back with the other fantail and comet, since this problem is genetic and not contagious. If you don't have another tank lying around, a large rubbermaid tub will work just as well. Wash with diluted soapy water, rinse very thoroughly with dechlorinated water, and that will serve as an excellent temporary home until the pond is warm enough for them to return.
You'll want to rehome the pleco or move him to another tank. He may try to take advantage of the fantail while she is resting on the bottom and suck on her slime coat. Plecos don't make good tankmates for goldfish either way - they have dietary and temperature conflicts, besides their taste for slime coat.
Next, perform a large double water change to remove as much medication as possible. Replace the carbon in your filter with fresh to soak up the residual. Feed the goldfish a high-quality sinking pelleted food, and feed the peas every few days to assist in digestion. You may need to try a few different pellets to find one that won't cause buoyancy problems. Since SBD is a genetic problem, each fish is different and can be floaty with one food, fine with another.
I suspect a larger tank, clean water and a sinking pellet will get the SBD under control, even if it doesn't solve the problem entirely.
I hope I'm reading your post correctly in that the 30 gallon tank is meant to be a temporary home until your pond warms up again. A comet goldfish should not be kept in a tank smaller than 55 gallons, and for all three of your fish I would recommend at least a 70 gallon tank.
Best of luck with her! Keep us updated.