30 liters is about 8 gallons, if you added the 3 catfish that would be about 16 inches of fish in there. Inch per gallon is not that accurate but it is a good way of quickly working it out. I think realistically you need a bigger tank or change your fish.
Pygmy Cories and Endlers are smaller versions of the fish that you have now and would work great in this size tank.
Wills