What kind of explanation are you looking for?
...one that perhaps would show itself upon the way they battle, move, search for food, etc. If they're 3-eyed, perhaps they see better, or have night -vision, or can spot you from miles away. This kind of explanation.
Incidentally, trolls are probably the most notoriois failure in the animation department. They're stiff as a submarine, frantically swinging their arms while the torso remais calm and collected like an English gentleman would.