Yes... a 3 week old tank is most likely still cycling...
You have WAY too many fish even for a fully cycled tank. you should have only like 5 fishes. Overstocking in a non-cycled tank is almost a certain death for all of the tank inhabitants. the steps you should follow:
1) order a master test kit
2) 50%+ daily water changes with prime
3) bring back as many fishes as you can to the fish store
if you dont do this your fish will inevitably die. You have 20 inches of fish in a 10 gallon tank and even if your tank was cycled this amount of fishes is double of the tank capacity.
in your profile it says you know about the
nitrogen cycle... so...