advantages and disadvantages of an accelerometer, gyrocospe and a magnetometer?

I reading about this topic, and I can't understand why are generally used this three sensors together. I mean, I know that each one has some advantages and disadvantages that are reduce working with this three components together but I don't know what they are. Can someone give me an idea?

For what I saw, even in some cases its only used the accelerometer and the magnetometer, without the gyroscope. How do the people to know what combination use? when they have what needs?

And the other hand, I read too that a magnetometer can be of 2 or 3 axes, and I don't understand what's the difference

These three sensors are used to form an IMU or Absolute Orientation Sensor, useful for drones, autonomous robots and the like.

When properly calibrated and configured, with good software to process the data, an IMU can be used to determine an object's orientation in space, relative to North and Down.

A good intro to the topic can be found at http://www.chrobotics.com/library