In structural engineering we are often interested in minimum and maximum principal stress. In NGSolve we are able to calculate the eigenstress by something like (assuming stresses to be a 3x3 matrix valued coefficient function):

eig_stresses = stresses.eig()

Then the first 9 entries of correspond to the stress eigenvectors and

eig_stresses[9], eig_stresses[10], eig_stresses[11] are the eigenvalues. However, it seemed to be that the order of the eigenvalues is not fixed. Is there an efficient procedure to order them into maximal, middle and minimal principal values?
In 2D I could use

Thanks for your help Michael.
Your solution works but for larger 3D models it is very slow.

It would be great if there is a solution which works faster and is implemented in the ngsolve environment. E.g. having .eig() already sorted or another option method .sortedEig()