It’s been years since we had a beta for any Call of Duty game, the last one was for World at War, and Sledgehammer is bringing it back with Black Ops 3 which is starting on the 19th of August 2015 for the PS4 only at the moment, with PC and XB1’s beta soon to follow at a later date.
Sadly the only way to get into the beta is by pre-ordering the game. Check out its website for more information.