Yes, that tank is definitely not big enough for the long term as fancy goldfish can grow to over 20 cm long without too much difficulty. I would also consider the filter on the not-quite-good-enough-at-picking-up-crap side of things for goldfish, an external would normally do a much better job. For the short term, the tank probably can take the three fish.
For the long term, I recommend you try to upgrade to a 120 cm tank, for example a 220-240 litre job. That would also allow you to keep more goldfish, while giving the existing ones more swimming space, and also the potential to add other species, such as weather loaches.