We formulate a perturbative framework for the flavor transformation of the standard three active neutrinos but with non-unitary flavor mixing matrix, a system which may be relevant for leptonic unitarity test. We use the $alpha$ parametrization of the non-unitary matrix and take its elements $alpha_{beta gamma}$ ($beta,gamma = e,mu,tau$) and the ratio $epsilon simeq Delta m^2_{21} / Delta m^2_{31}$ as the small expansion parameters. Qualitatively new two features that hold in all the oscillation channels are uncovered in the probability formula obtained to first order in the expansion: (1) The phases of the complex $alpha$ elements always come in into the observable in the particular combination with the $ u$SM CP phase $delta$ in the form $[e^{- i delta } bar{alpha}_{mu e}, ~e^{ - i delta} bar{alpha}_{tau e}, ~bar{alpha}_{tau mu}]$ under the PDG convention of unitary $ u$SM mixing matrix. (2) The diagonal $alpha$ parameters appear in particular combinations $left( a/b - 1 right) alpha_{ee} + alpha_{mu mu}$ and $alpha_{mu mu} - alpha_{tau tau}$, where $a$ and $b$ denote, respectively, the matter potential due to CC and NC reactions. This property holds only in the unitary evolution part of the probability, and there is no such feature in the genuine non-unitary part, while the $delta$ - $alpha$ parameter phase correlation exists for both. The reason for such remarkable stability of the phase correlation is discussed.
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