treating a "sick" fish in a tank where there's an inherant problem rarely works out well. if you don't fix the problem first, the 'cause' of that white spot, no amount of drug treatment will be effective in the long run. the weakened fish will only get sick again and the second time, may not take treatment well.
healthy fish do not succumb readily to disease. stressed and weakened fish do come down with 'this and that' quite easily. have you checked your ammonia, nitrite and nitrate? what's your pH? what's the water temperature? aside from the fact that you've got 3 gold fish in a tank which is far too small for them, what other stressors can you find?
you cannot expect to maintain 3 goldfish in ten gallons of water. goldfish are naturally messy and exhaust a high level of ammonia compared with other fish. they therefor need larger tanks than other fish of the same size.
one small goldfish (3-4 inches) needs ten gallons by itself. three of them need about 40 gallons. if your goldfish are larger than that, they need an even larger tank.