He sounds more like he's suffering from stress than any type of illness. Also, something to remember, the color of the poop will vary according to what he's eating. White poop does not always mean the fish is sick or has a parasite.
How big are these fish? Size of the tank will mean nothing without also knowing how big each fish is. Oscars need territory, and if 2 of them should pair up, the 3rd will be bullied and forced out of their territory... the symptoms you describe sounds just like that... more space and watch those water params... oscars are horribly dirty fish. My albino is now almost 12 yrs old, his quarantine tank alone is 75 gallons, his main tank is 225 with 1 other oscar and its full!! Whether that fish is truly sick or not, you might want to consider a quarantine tank for him. Also, how much decoration is in the tank? If there isn't enough, then fighting for territory will get worse, and fast. These guys need a lot of space to be happy, and an unhappy oscar lays around and does nothing all the time. It's quite obvious.