No, 3 goldfish in a 50 gallon tank is NOT OVERSTOCKED
. You could keep 4-5 fishes in it without problems
However, cleaning out the whole system is very, very bad. You're constantly exposing your fish to ammonia, and that's most likely what killed your moor and what is about to kill your other fish. You need to buy a gravel vacuum and start cleaning the gravel with that, take out about 50% of water every week, your tank isn't very big, so it needs weekly water changes (any tank does, basically, except for monster tanks).
You should also buy a water test kit for ammonia, nitrite, nitrate and pH and test it every single day. If you can't do it, ask the person who cares for your fish, and DO NOT LET THEM FEED THE FISH AT ALL. You're the only one who should do that when you're home, and only once, and only a small, small amount or the tank will get polluted with ammonia again.
You need to let the system cycle, the best way to do that, is by monitoring the water quality and changing the water when the waste is in too high concentrations, ammonia for instance is very harmful in small concentrations, so I'd change it as soon as it reaches 0.5ppm. And again, ONLY 50% of the water!!
Buy some beneficial bacteria, it's usually sold in all pet shops, so just look for one that fits you
When I cycled my 83 gal, I used "fluid bio filter media" by Easylife, and my tank cycled in two weeks. This conditioner will also protect and strengthen your fish, and you don't need to use any other while you use this. There is also Stability by Seachem that I know people on this forum have also used successfully.
During the cycling process, you'll notice the ammonia slowly disappearing and nitrite showing up, this is still toxic, but not as dangerous as ammonia, so keep up with the water testing and water changes! After the nitrite go away, nitrates will show up, and these will gradually increase constantly and needs to be controlled with weekly water changes
After the tank has cycled, you should test the water weekly before water changes, and add beneficial bacteria to keep the ecology in the tank healthy and working
Your fish will probably perk up once you start changing the water, so if you can't buy the test kit and beneficial bacteria NOW, you can still change 40-50% of the water every second day until you're able to. BUT YOU CANNOT SKIP IT AND RELY ON WATER CHANGING ONLY!!!
So, good luck now, and keep us updated! Ask if you have any questions