PC only: If you own Black Ops 2, Ghosts, or Advanced Warfare on Steam, you get access to BO3Beta

Players on Steam who own either Call of Duty: Ghosts, Black Ops 2, or Advanced Warfare, can get access to the Call of Duty: Black Ops 3 multiplayer beta now. The beta should appear in your downloads list if you own any of those games – no code or preorder is required. 

SOURCE: Steam (via @WePlayCOD)