@aurora113, try the below possible solutions one-by-one:

• Try restarting your computer and then verifying the integrity of the game cache for Arma 3.

1. Restart your computer and launch Steam
2. From the Library section, right-click on the game and select Properties from the menu.
3. Select the Local files tab and click the Verify integrity of game cache... button.
4. Steam will verify the game's files - this process may take several minutes.

• Try all of the possible solutions listed on the Arma 3 Unusual process exit article on the Bohemia Interactive Community Wiki.

If the issue persists after trying all of the above possible solutions then use the Arma 3 Launcher to prepare a troubleshooting report and then upload the report to the feedback tracker for the developers to review.

For issues regarding Arma 3 you can file a troubleshooting report, also via an action available in the option menu in the upper right corner (gear icon)

Be sure to upload the whole report file. If the report file will not upload directly to the feedback tracker then try uploading to a free file sharing service such as Dropbox, Microsoft OneDrive, Google Drive, Mega.nz, and so forth, and then link to the report file.