It's easy you just have to creat a mission.biedi file ( file format for 3d editor) and enter this in a shortcut "C:\Program Files (x86)\Steam\SteamApps\common\Arma 3\arma3.exe" "C:\Program Files (x86)\Steam\SteamApps\common\Arma 3\...\mission.biedi". ... is for the folder you put the mission file.