Welcome to the forums! The causes of dropsy are often difficult to diagnose... so I'm not sure what to suggest there, other than keeping up with water changes, there are a few posts here about dropsy if you have a look for them and perhaps peoples previous experiences can help you..... but I also need to mention that your tank is heavily overstocked.
The absolute minimum stocking for fancy goldfish is 20 gallons for the first fish and then 10 gallons for each additional fish.
Another stocking rule, that I prefer is 26 gallons/100 litres for the first fish and then 15 gallons/60 litres for each additional fish, it generally just gives the fish more space which they will appreciate
So for keeping 3 fancy goldfish, you need a tank that's between 40 - 56 gallons, but with a plex and cory cat in the mix, I would say you'd need something bigger to accommodate them as well, so maybe something around 60 or 70 gallons.
The reason for such large tanks being needed is that fancy goldfish can grow very large, usually around 8 - 12 inches depending on the breed, so they need lots of space to be able to grow and exercise properly -although there is a chance your fish may have stunted growth, which means they will grow slower, seeing as they've been kept in a small tank.
The other reason for large tanks (and also large filtration) is to keep up with the waste these animals create. In a small tank waste can build up to harmful levels, especially as the fish get bigger and can cause problems with their health.
I would suggest to read this slowly and carefully and take your time to think things over, it is obvious you care greatly for these fish and only want the best for them
There are often second hand tanks around which can be cheaper than buying brand new, or you could always look for someone who has a tank large enough if you were unable to buy such a large tank - which would be completely understandable, it is a lot to expect someone to just jump up, run out and be able to buy a massive tank.
We are more then happy to help if you have further questions or need further advice
Do let us know how little Bella is doing.