Every time you exit the Esc menu, targets and objectives are highlighted briefly. I can't find the shortcut to do the same. Is there one?
This is depending on your difficulty settings.
There is no key, that you can use to show them directly.
While "veteran" or "expert" won't show you these overlays, "recruit" will show them every time. On "regular" they'll only show if you hit escape and get back into the game.
However I think you're playing Arma3 nowadays anyway. But it's really the same there. :)