Does a 3-year PhD experience in a non-German EU university count in determining the salary for a TVL-13 (100%) position?
I worked for three different universities in the same Bundesland (state) in Germany. Although they all followed the same general regulations, the interpretation was rather different.

one considered my PhD (on a scholarship, so not employed) as relevant experience, one not.
one said my Cambridge University (UK) degree was a Fachhochschulabschluss, one considered it as full university degree.
one was willing to negotiate (not with me, but with the institute that I should work for), one was not.

So you need to talk to your future boss and to the university that wants to employ you.