A BMW X3 does not have a dipstick. Their is no way to check the transmission fluid on a sealed system. You can add fluid to your system by pumping into the fluid input hole on the transmission itself. DO NOT LISTEN TO Opaul75, he has no knowledge of how this transmission works. His advice is for a standard type domestic transmission.
You can check the transmission fluid on your vehicle by locating the dipstick handle sticking out of your transmission, then pulling out the dipstick followed by checking the fluid. Before performing this task, ensure the vehicle is parked on a level surface. It is recommended to check the transmission fluid once a month.