The Arma 3 alpha has concluded, and closed beta testing is now underway on Steam, accessible to anyone who pre-purchases Bohemia Interactive's military shooter sequel through Steam's Early Access program. There is a Steam Community Announcement about this with details on the game and the beta, and here's a bit:

Supporting the development of its upcoming tactical military shooter, Bohemia Interactive has released the anticipated Arma 3 Beta. The Arma 3 Beta expands the game’s diversity by focusing upon heavy firepower and military logistics. Combined with the playable content from the Arma 3 Alpha, the complete package features the 20 km² island of Stratis, 20 weapons, 18 vehicles, 3 factions, 8 showcase missions, 4 multiplayer scenarios, 4 challenges, the scenario editor and modding support.