Take the catfish AND the birchirs back to the shop, 3' tank is to small for either fish, the catfish grows bigger than the tank, and the birchirs, assuming they are senegalus, the smallest birchir there is will also grow to around a foot long needing minimum of a 4' tank.
Top tip, DO NOT LISTEN TO ANYTHING THE FISH SHOP TELLS YOU, they will lie out their rear end just to get a sale.
Take them back before they die a horrible death.