Two male will confirm fight one... You can try for a 6ft tank if your mandarin are not too big becos in a 6ft they will rarely bump into each other...
But better stick to your pair... lol... Prefer the spotted one rather than the green mandarin... lol..
It will be difficult to confirm, as it might be a S. nematodes that broke its dorsal during shipment... Cos there is not much difference other than the dorsal fin bah...