Nitrite eating bacteria takes longer to establish than the ammonia eating bacteria, sounds like your tank wasn't quite cycled.
Doing large water changes is definitely the way to go. Are you adding Prime as well? That will detox the ammonia and nitrite, making it safer for the fish.
I might as well ask because it is going to be asked at some point, why 3 fish in a 30? Is this their permanent home or temporary? The minimum number of discus is 5 or 6.