Here is one tracking technique I have always liked.
This example is 2D, but can be extended to 3D easily.
Picture 2 light sensors, a fraction of an inch apart on a flat panel, left and right, and with a playing card separating them.
Lets assume you have motors that can cause the face of the panel to point more to the left or to the right.
If a light source is directly in line with the playing card, equal light shines on both sensors.
If the shadow from the card starts to cover the left sensor, increase drive to the right pointing motor.
If the shadow from the card begins to cover the right sensor, increse drive to the left pointing motor.
For 3D tracking, implement the same for up and down directions.
Turn off all motors when both sensors show dark.