Problem :
Game crashes after 3-5 seconds in main menu. ( Steam Workshop = unused )
My solution ( works for me ) :
1) Start Steam in offline mode.
2) Start ArmA3 ( it shouldn't crash now ).
3) Go to main menu / play / showcases.
4) Start all showcases you have played ( doesn't matter if completed or not ).
5) If you get an error message like "couldn't load savegame / wrong version" , press "cancel" then "delete" ( do this on all showcases you've played ).
Now it should work again. The old savegames seem to be incompatible with the latest version. Game crashes while loading them on start.
This worked for me.