Hello, all! My name is Lauri and I'm happy to find this forum!!
I currently have 2 tanks. A 5 g tank at work with 3 goldfish in it (one will be moving to a friends tank) and a 18 g tank I just bought today (I LOVE Craigslist) and have set up and ready for fish!
I'm sorry to tell you this but 3 goldfish need around 40 gallons. They get big and produce a lot of waste.
And you might think the tank is ready for fish but you need to cycle it first. It takes about a month without any seeded filter media.
Someone will probably be here to give you the appropriate links to cycling instructions soon.