Drawing a mask based on three vertices obtained as a result of 3D Tracker

The trackers in Camera Tracker are added to small areas of detail, don't last for the entire shot, are not numbered, and contain no usable depth information until at least 3 of them are selected to define a 3D plane. If you have a camera pan, there is no depth information.

My procedure for using the Camera Tracker is always to establish an Origin and Ground plane with at least three trackers, then add a Camera and a Solid to the scene using the same trackers. If the shot is not a camera pan, you will now have depth information for every tracker. Once you are there, if you want to mark a single tracker, you can select just one tracker and create a new solid (I don't use nulls because you can't see if the track is accurate). You'll get a small solid, and you can see if it accurately tracks the feature. There is no accuracy calculation for individual trackers, so you must run a preview and look at the movement of the new solid to see if it accurately tracks the feature with the solid. 

When I get that far, I set all of the solids that I have added using Camera Tracker to Guide Layers and start adding my 3D elements. You could easily pick any of these solids and use them to generate a 2D vector path and keep it stuck to the tracked marker by creating a simple path with the same number of points as the trackers you want to follow. If you want to follow three trackers, create a closed path for a solid shape or complete mask, or 3 points on an open path you want to use to draw a line or create a motion path.  When you have the path created, select it and open Create Nulls From Paths. jsx from the Window menu, and apply Points Follow Nulls. You can now Shift + parent the path nulls to the three-track solids. 

The last step is to put thee track solids above the nulls that were created with the script and add this expression to the position property of each null created by the script. 

p = thisComp.layer(index-1).toComp(anchorPoint)

 You will end up with a comp that looks something like this project I recently created:

I'm sure the command IDs are listed somewhere, but even then you'd have to instate a custom event listener to derive the context. This UI-based stuff doesn't have generalized hooks in the script engine and since the tracker technically is a plug-in, there may be some extra caveats to intercept those commands.

Mylenium

Of course you would need to implement any proximity checks for unselected points yourself. That's a non-trivial math problem completely nothing to do with AE. You need to write your own vector math.

Mylenium

You need to work with layer space transforms like fromWorld() and toComp() or create your own code for the matrices.

Mylenium

The question requires clarification. Can I select specific 3D points obtained from the 3D Camera Tracker plugin's point array. For example 5, 12, 55 or 8, 32, 99