1. Place a vehicle, name it: myVehicle
2. Add the getIn EH to the vehicle:
myVehicle addEventHandler ["GetIn", {
_vehiclePos = _this select 1;
if (_vehiclePos == "driver") then {hint 'entered vehicle as "DRIVER"'};
if (_vehiclePos == "gunner") then {hint 'entered vehicle as "GUNNER"'};
if (_vehiclePos == "commander") then {hint 'entered vehicle as "COMMANDER"'};
}];
3. Enter as commander, gunner, driver