Hallo i need a help to find key from the door. Quest begins in book with some legend. There are 3 dungeons with parts of amulet. Two of them are opened. 1 is locked. Written that key is needed to open. Thank you.

the one that is locked can only be accessed by starting the College of Winderhold. The first quest for them will get you inside.

I was thinking about it. Thank you again.

Yes. It's part of Bethesda's master plan to ruin all role-playing by requiring the player to join every faction in the game, whether it makes sense for the character or not. Just wait till you try to find the stones.