I’m sure there is an expression you could use to find the real world space and orientation of the master null, but I will have to leave that up to the experts. Also, an expression can be based on conditions like frame number, which would allow you to choose which null to use to claim data from.
But try this. Make the master null and footage layer 3D layers, and create a new camera. Move to the last frame of the comp and parent the camera to the master null. Re-frame the camera appropriately by adjusting the z-rotation and zoom level. The footage is now stabalized.
But is this what you were looking to do? Or are you just trying to smooth your pan?
BTW – Correct me if I’m wrong, but you don’t need to seperate the track into three nulls. You can simply adjust the spline in Mocha as needed and it will continue to maintain the tracking data.