Yes, Snapz is right that actors should be exported one by one, and then combined in 3D editor.
As to cameras, it is very important to have more views to minimize mutual occlusion of actors.
That's why PS3 Eye cameras are much more recommended.
With Kinects you can track only 2 actoras and relatively simple motions when actor are not close to each other (like handshake). If one actor occludes another in one sensor, you'll get tracking error.
Also, you'll have a separate computer for each Kinect v2.
As to PS3 Eye cameras, we recommend to use at least 4 cameras per actor. So 12 cameras is good enough for 3 actors.
If you do not have super-powerful modern desktop, you'll have to use 2 computers for recording.
More details can be found here:
http://docs.ipisoft.com/Multiple_Actors