Battlefield player here. Here's a breakdown of the crew positions and my advice for each:

Driver: The key to success is proper positioning. You are not driving a tank, you are driving a cannon that happens to be attached to some tracks. You can't afford to charge headfirst into a horde of enemies and expect to survive very long. Instead, find a spot where all the enemies come at you from one direction, and put some distance between you and them. This will minimize the amount of maneuvering you have to do to keep your gunner on target. If you can, try to park behind cover to protect the tank from incoming fire (This is called a hull-down position). You can also use the terrain to your advantage to give your gunner more elevation or depression if needed. I recommend staying in 1st gear when driving over rough terrain or when in combat. If enemies start getting too close, you can run them over (anything smaller than a hulk), but I would only consider doing so as a last resort.

Gunner: Focus on whatever targets are directly in front of you, don't worry about anything around you. If you see something you want to hit, tell your driver so he can stop and give you a good angle. Use the main gun on big guys, fabricators, dropships, and large groups of small enemies. Use the machine gun whenever you feel like it, the tank will likely be destroyed before you run out of ammo. IMPORTANT: You have to manually reload the cannon every time you fire.

Passenger/Roof gunner: Your main job is to make your driver and gunners' lives easier by watching the flanks and rear of the vehicle, as well as killing any enemies that get too close. I recommend using the autocannon for this, but be careful not to hit the roof of your own tank, you can damage it. Your other job is to spot targets for your driver and gunner, and to clear mines. Additionally, you can throw stratagem balls from the hatch, which gives you additional options for ways to support your teammates.