3 unknowns but only 2 equations: Iterative approach?

The problem consists of 3 intervals (m) (they are called steps in the data below because the problem represents a physical glued joint with 3 steps (ie 3 different thicknesses) of length Lstep. The numerical problem I need to solve is finding constant A1 and A2 for each step (knowing a number of inputs). I was provided with the “raw” date for the solved problem and it seems that the constant A1 and A2 were found by an iterative process. For each step with length Lstep an initial (random?) value TR, at right hand end of a step together with the value at the left hand end (TL = T given at x=0) and does an Iterative process to calculate A1 and A2.

There are 3 unknowns but only 2 equations, Is there an easy (built-in!) iterative approach that can be used to find the value of TR, A1 and A2?

Thanks

Regards

JXB