Calculating Lessons available based on info from separate apps

Trying to build a calculation field which pulls from 3 separate apps and can't quite figure it out.

App 1 - "Students"

App 2 - "Payment Center"

App 3 - "Lesson Tracker"

The "Students" app is home base and tracks all info related to my students. 

There is a workflow to 'create a payment' which utilizes a "Payment Center" app to record all payment info on a relationship to the Student name in "Students" app.

In the "Payment Center" I fill out how many lessons this payment covers.

Each time the student comes for a lesson, I use a workflow to 'track a lesson' which creates an item in the Lesson Tracker app referenced via a relationship field on student name again.

NOW - I would like to build a calculation in the "Student" app that tells me how many remaining lessons that student has paid for.
aka it adds every time a payment is referenced to that student and subtracts every time a lesson is referenced to that student. I can't figure out how to connect it across all three apps. 

Any help would be greatly appreciated.