I think you're tank may already be overstocked as it is just with the 3 goldfish.. But I am not a goldfish expert, so I guess it would have to depend on what type of goldfish you had?? I do know that most get pretty large, and they are POOP MACHINES!!!
yup you need to either get rid of the goldfish or corys as goldfish are coldwater and corys are tropical and for one goldfish you need 20 gallons of water and every one after you need another 10 gallons so get rid of the goldfish and then restock your tank with some small fish as it is overstocked right now and with just the corys it will be almost overstocked or you can get a bigger tank