Fifteen gallons is not much for goldfish. These fancy goldfish can easily reach 5-7 inches and are very messy fish just like thier common predecessors. Without enough filtration you will have a very murky globe.
On the plus side, and lucky for you these are slow growers so that you would probably be able to have 3 in there for a span of about 2 years. Just be careful, the thin skin that makes up the bubble can easily be popped, and even though it will grow back, you do not want one that has a large bubble on one side, and a small on the other. so watch the decorations, and also that you don't put to many fish in that will go after each other.
best of luck.
btw these fish can live for a very long time.
do some research before purchasing.
