  We present the frequency redistribution function for the polarized three-term
atom of the $\Lambda$-type in the collisionless regime, and we specialize it to
the case where both the initial and final terms of the three-state transition
are metastable (i.e., with infinitely sharp levels). This redistribution
function represents a generalization of the well-known $R_{\rm II}$ function to
the case where the lower terms of the transition can be polarized and carry
atomic coherence, and it can be applied to the investigation of polarized line
formation in tenuous plasmas, where collisional rates may be low enough that
anisotropy induced atomic polarization survives even in the case of metastable
levels.
