Addendum: Improved MSSM Higgs mass calculation using the 3-loop FlexibleEFTHiggs approach including $x_t$-resummation

In this addendum we present the stand-alone C++ program MSSMEFTHiggs3L, which implements the 3-loop FlexibleEFTHiggs approach to calculate the lightest CP-even Higgs boson pole mass in the real MSSM at N${}^3$LL and N${}^3$LO with $x_q$ resummation, presented in JHEP 07 (2020) 197 (arXiv:2003.04639).