The word troll comes from the Norse mythological epics. There is no real troll even there. To some up the original definition of a troll "A mythical creature that usually has one or two heads, but sometimes more, that isn't classified as an undead, dragon, serpent, giant, dwarf, goblin, or elf. They range from being covered in bark, being the most beautiful creatures capable of seducing any man or woman, or two (or more) headed monstrosities that eat human flesh (and sometimes they don't)." All around troll could be translated into monster, we just don't because someone way back when thought troll sounded like toll and stuck one under a bridge which had personified goats crossing it. With the definition of monster one can take it anywhere they want to really. So why not a three eyed ape-like creature that has a taste for human flesh (and alcohol if you ever saw the suicide troll). Figuring you wrote this because it seems off because nature usually comes in twos and not threes, then just imagine the third eye is some kind of symbolic marking. Besides that, I can't really agree that they should retcon there troll which has been established since Redgaurd. Its three eyed skull even cameoed in Morrowind on the Troll Bone Helm.
Norwegian folklore is very rich of trolls. Almost half of the mountains in the country are named after trolls and have old stories bound to them.
Norse mythology sources speaks mostly about Jotuns, which, while having several things in common with the trolls, are in general far smarter and not so ugly.
Trolls are not too uncommon in the fornalder norse sagas. Usually living in caves or abonded houses in the woods. Trollwomen are mentioned most, and usually depictured as evil, very strong and ugly.
Rarely do they have more then two eyes or one head though.
They are also quite common in Norwegian folklore all the way up to 1800.
Bergrise and Jutul are two other names for jotuns, also common in Norwegian folklore(usually depictured more as giant trolls).