Welcome to AA
I'm sorry to tell you...
You're going to have to change water twice a day to keep fish alive till you can return them. Goldfish need Big tanks or ponds. I hope you have good water conditioner.
Do an immediate large water change.
Test the water. Post results, we'll try to help.
Absolute minimum for Fancies ?
20g first fish and 10-20g per each additional.
Comets/Commons need ponds.
Please return or rehome the fish.
A 3g can have perhaps a single Betta
you can have plants and Cherry shrimp ... No fish.
The GF will suffer and die in such a small tank. Ammonia poisoning.
I'm very sorry the store did not give you tips on proper Goldfish (GF) care.
Fancies can live over 15yrs
Commons can live over 25yrs.
They need space to grow. They are big fish.
If you want a forever tank for 3 Fancy GF ? Look at 75-100g tanks with
Canister filters rated at 2-4x tank volume.
Smoke signals from my iPhone using Tapatalk