3 small fish in a 12 is easy done provided you keep the water changes up to it.
Keep an eye on your fish and you will soon tell whether they need a water change, but we keep 2 O. Clowns and a small bi colour blenny in a 6-7g with a leather, brain and a couple other
hardy corals with no problem. I change half the water once ever 2-4 weeks depending on how things are looking and if the missus has over fed the tank ( happens a bit

).
We also keep another nano, about the same size, with a family of domino Damsels, five in total, and an anemome that is growing at a decent rate. The tank may have to be swapped with our larger one, purely because the anenome is slowly outgrowing the tank (its about 12 gallons too i think). But the point is, with small fish and good husbandry, you shouldnt be too scared of the 'general' stocking rules. The damsel tank has a 30-40% water change fortnightly.
Just make sure you dont get slack with the water changes!
