Fancy goldfish are usually best kept in a tank only with other fancy goldfish although some people are successful in keeping one of the dwarf plecos like a Bristlenose and other keep some cool/cold water fish like Whitecloud Mountain Minnows, Zebra Danios, Buenos Aires Tetras, and others.
The main thing is to fishless cycle the tank first since goldfish put out so much ammonia and waste that they are not the best fish to cycle a tank with and the high ammonia/nitrite levels during "cycling with fish" can cause long term health issues.
Of course, you should have a BIG tank for goldfish. As a startup tank, a 20G long tank for the first and at least 10G more for each additional goldfish but I personally think a 4' long 55G should be the smallest start up tank which will be good for a few years for 3 fancy goldfish. Then it would need to be upgraded as well.
