Welcome to the forum and you have come to the right place for answers.
First and very important thing I think you all ready know is don't trust anything the, "Pet Whatever," store tells you. The current setup will not house goldfish very long, but it was the right move to make for now. Cycling that small of a tank with 3 goldies in is equal to cycling the same tank with 25 guppys. The goldfish have a huge bioload so a 10%-25% water change daily is needed after the tank sits as is for 3-4 days.
Once you are over that tank cycling bump in the road it is time to do one of two things: Wait for the Petco $1 per gallon and get a 55gal or start looking on craigslist for a 55gal or larger set up. The unfortunately the reality is unless you want to do 25%-50% daily water changes the rest of your life, I'm guessing not, even those tanks will be too small to hold two comets-feeder goldfish. Within a year the should grow to 4"-6" depending on food and water quality. Even if you do a bad job on both of those aspects, comets just keep on growning. Mine started as dither fish for my Jack Dempseys and now are twice as big as the Jacks in just two years. Fortunately for me the owner of a pond close to my house said I can release them there next spring. If you just keep the fantail you can get away with a 30H and a lot of water changes. I'd add a sponge filter rated for 125gal and a small HOB
that allows you to reload your own filter cartridges. I'd use foam pad and a mesh bag full of carbon.
If you decide Mr. Goldfish needs to go then come back here and ask about tank stocking. There is never a shortage of opinions and the design and fish selection can be a fun experience. Again, no, "Pet Whatever."