Посмотри здесь,
http://feedback.arma3.com/view.php?id=1685
И попробуй это:
1. Turn on Hidden Files and Folders in Windows
2. Navigate to: C:\Users\*username*\AppData\Local
3. Delete the folder labeled: "Arma 3 Alpha"
4. Open Steam
5. Right Click Arma 3 Alpha
6. Click Properties
7. Click Local Files
8. Click Browse Local Files.
9. Right click Arma3.exe
10. Click Properties
11. Click Compatibility
12. Check the "Run as Administrator" box
13. Click OK
14. Run Arma 3 from steam, or from exe.