In my many years in this hobby I have found the higher the temp the more active the fish become. A temp between 78-79 is perfect for your fish. I have Discus and most die hard Discus breeders keep their tanks in the higher 80's. Not me, I keep mine at 78.
You should notice a difference with a lower temp plus like mentioned 3's a crowd and one will definatley get picked on. If you can buy 2 more the same size it will spread out the agression better.
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Bugsy.... 
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