In arma 2, player as leader of some units can order a unit to get in a vehicle such a tank as commander/gunner/driver by selecting him by F keys, then click on a vehicle, then immediately click again, so AI will be ordered to get in same vehicle as another rule.
1-you select your unit and order him to mount on a tank by pointing mouse on that tank.
2-he will be ordered to get in as "driver" for example.
3-then if you immediately select him and order him again to get in that tank, he will should be ordered to get in as gunner this time, as was in arma 2, but this simple feature is not available in arma 3 and player has to take more time and order unit via commanding menus.
please bring back this feature as it help players to order their AI units much faster and easier.