The last goldfish I had grew from under 1" to 8" in about 7 months, and that growth rate seems common in decent sized tanks/warm ponds. If your goldfish have been in an indoors tank for 2 years and aren't around a foot yet, they are clearly quite stunted.
2 years is nothing for goldfish, goldfish are just a species of carp - they are big, messy fish, and can live to over 40.
On the tropical side, it's no different from trying to keep 3 silver sharks/plecs/oscars or whatever other foot-long species in a 10g. You can do it, and keep them alive, but not healthy or unfortunately even "fine".
They will be far more likely to succumb to water quality issues, disease and various possible deformations from the stunting. Added to this, most goldfish nowadays aren't as hardy as they used to be, often being kept and bred in very unhealthy conditions before purchase.
Where are they now, btw?