I agree with fraservet and Chac
I didnt see how long your tank had been set up, but most people would be hesitant to recommend even one mandarin in a tank that is still in its early stages. Mandarins should on be kept in mature tanks with lots of live rock. I.E 75 gal tank 100 lbs of LR.
Depending on tank size you could have clowns with your problems. Err... I mean problems with your clowns. As of now (unless you bought adult paird off clowns) your clowns are all sex-less juvenilles. As they establish their home in your tank they will go into a seemingly endless bout for dominance (while some are short, this process can last several months). This bout is important for them in several ways. The strongest one will grow larger than the other two and become the female. The second will become the male, and the third will remain a sex-less juvenille all its life (unless on of the other two dies for some reason). The problem with keeping three is that in a small tank (once again Im not sure how big you have) the one that essentially lost the dominance struggle will be harrassed and bullied unless he can escape. And that means he will try to get away, even if the only place to go is up and out of your tank. Or the stress could kill him. SOOOO...if you want to wait for them to pair up then return the third great, otherwise I would return the third NOW, instead of waiting for what might be a disaster in their little world.
Also - Everyone Ive talked to about setting up tanks has always advised to keep the number of fish that occupy the same niche to 1 unless they are very very placid. Just make sure you know what your getting into when you have two blennys, and two damsils (plus ive heard that damsils are hard as hell to catch when you want to get them out).
Good luck on your tank.
-edit- Just did the maths on the 270 liters - approx. 80 us gallons - you can keep the third clown, if thats what you want to do, but make sure there are lots of hiding places for him.
