  We present some results on the analysis of three pion resonances. The
analyses are motivated by the recent release of the largest data set on
diffractively produced three pions by the COMPASS collaboration. We construct
reaction amplitudes that satisfy fundamental $S$-matrix principles, which
allows the use of models that have physical constraints to be used in fitting
data. The models are motivated by the isobar model that satisfy unitarity
constraints. The model consist of a Deck production amplitude with which final
state interactions are constrained by unitarity. We employ the isobar model
where two of the pions form a quasi-stable particle. The analysis is performed
in the high-energy, single Regge limit. We specifically discuss the examples of
the three pion $J^{PC}=2^{-+}$ resonance in the $\rho\pi$ and $f_2\pi$
channels.
