Arma 3 is set to launch its Alpha test on March 5 for those who pre-ordered the game. You'll be able to play four single-player showcase missions, two multiplayer scenarios, and tinker with the scenario editor and modding.
The official site also notes that, for those who don't mind some waiting and compromise, the "Alpha Lite" version will be available on March 12. It is invite only, and doesn't support multiplayer or modding, but it will still give you a taste of the game. Bohemia has also set up a system requirements page.
As reported last week, Arma 3 will be exclusive to Steam. Project lead Joris-Jan van't Land said if not for the Steam exclusivity, the game may not be able to come out this year at all.