The Call of Duty: Black Ops III website now offers word that beta testing for the Windows edition of the upcoming military shooter sequel will begin on August 26th. The Xbox One beta begins then as well, which is a week after testing commences on the PlayStation 4. You can get in my preordering, though they warn in a FAQ that "Each retailer will have a limited supply of Call of Duty: Black Ops III Beta codes, and will be available while quantities last." They also further define the scope of testing, which sounds like it will only run for four days:

PlayStation 4 owners get access to the multiplayer beta first starting on Wednesday, August 19 and will continue through Sunday, August 23. Then, the following week on Wednesday, August 26 the beta will make its debut on Xbox One and PC and run through Sunday, August 30. During the beta, fans will have an opportunity to try the new momentum-based, chained movement system, find the perfect Specialist to fit their play style, get a feel for the new multiplayer maps, and more. Actual platform availability and launch date(s) subject to change.