Hi,
There is really no "soft" way to put this so I am just going to "say it." that tanks is too small for comet goldfish, they belong in a pond. A few things could happen (and likly will happen) in this instance. The big fish has been stunted and grown only "to the size of your tank." If he were in a pond he would be huge (over a foot long). Stunting does cause damage to fish and now that you have added the other two it is likely for those problems to surface.
Therefore:
1. Your big fish will die from stress of too many fish in his tank even if they are small. He is very susceptible because of the stunting, which is why I put this out as the first scenario.
2. The smaller fish will survive until they too start to stunt then at least one will die.
3. The other one may or may not survive to be about the size of your original fish. It will be stunted and unhealthy and therefore susceptible to problems.
4. They will just all die.
If this were me I would want to find a more appropriate home for my fishes. I would give the 2 I just got back to the store I bought it from, and then never shop there again as they clearly are not caring about the lives of their animals if they think the goldfish will be "okay" in a tank that size.
Then I would research what I could keep in a tank that size and get some happy fish
Again, that is what *I* would do. These are your fish so you have to decide what the priorities are going to be for them and their lives.