I think 15 liters is about 3 gallons...I'm terrible at conversions lol. Anyway, there's a lot of misinformation about goldfish. Goldfish haven't gotten any more complicated lol- they've always been a little bit of a hand full if taken care of properly. I'm not sure what kind of goldfish you have (or had rather), but there's no breed of goldfish that I would keep in a tank smaller than 40 gallons. I wouldn't keep a goldfish in a 3 gallon container for even a moment. Goldfish actually grow to between 8 and 12+ inches depending on the breed. I have comets and shubunkins in my 1200 gallon pond and I believe my largest fish is 15 inches. I originally got him from a carnival when I was 6 lol. I'm 23 now which gives you a bit of an idea of how much longer they can live compared to their commonly believed life span of maybe a few months. They can actually live about 30 years. If they're kept in tanks that are too small, they die. Goldfish put out a huge bio load and it's WAAAYYYY too much for a 3 gallon tank to handle - even a baby goldfish will overload the tank almost right away and die. The best way to start off if you're interested in goldfish is to buy a tank big enough for them when they're adults. It's cheaper that way too. Some people will insist that they'll just upgrade when it's time, but usually they miss that time and the fish dies. Even if they do upgrade they end up paying for two or even more tanks rather than just one.
So if you have something like a fancy goldfish, start with a 40 gallon tank and then add 10-20 gallons for each fish (so 60 for 2 and so on) and if you have a comet (often sold as feeder fish) start with a 55 and add 20 gallons for each additional fish. I just helped my friend set up a super cheap total frat guy pond in his basement lol. We picked up a hard plastic kiddie pool from the hardware store ($15), spray painted it with Krylon Fusion spray paint ($3 - it's fish safe), made sure to let it cure for a few days, then added water, gravel (free from some guy we met), some pond plants ($25), and finally 4 comet goldfish (free because I gave him some of mine but super cheap in the store). The pool/pond is about 150 gallons, so it's just right for 4 comet goldfish. The only expensive part can come in with filtration. As I said, goldfish are super messy, so they need their water turned over at least 10 times an hour! So for a 150 gallon "pond", we got a water pump rated at 1500 gallons per hour. I think it cost about $60. Then I helped him make his own filter using a $2 5 gallon bucket, some 100% polyester stuffing I pulled from a pillow, some pot scrubs and filter pads I had lying around, and some ceramic media I also had but someone's selling four bags for $15 on ebay. You can find videos on youtube about making filters for tanks of all sizes.
I'm sure there's a lot more to say, but I can't think of anything right now. If you're really stuck with a 3 gallon though, then the only things I would recommend are shrimp and snails. Goldfish just don't belong anywhere near a 3 gallon container. Sorry
10 gallon tanks usually aren't very expensive though. I pull some off craigslist for $5-$10 when I need more. The 10 gallon tanks that I've actually used as permanent homes for fish were fun
It's the perfect size for a betta. I'm about to get a betta myself and add him to a 10 gallon I have with an african dwarf frog, some cherry shrimp, and a couple assassin snails. Guppies, dwarf corys, a couple kinds of gobies - I think maybe bumble bee - are some fish that are ok for a 10 gallon too.
