incompressibleThreePhaseMixture: Corrected the construction of the third-phase properties

applications/solvers/multiphase/interMixingFoam/incompressibleThreePhaseMixture/threePhaseMixture.C

-/*---------------------------------------------------------------------------*\
+u/*---------------------------------------------------------------------------*\
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
@@ -80,7 +80,7 @@ Foam::threePhaseMixture::threePhaseMixture
         viscosityModel::New
         (
             "nu3",
-            subDict(phase2Name_),
+            subDict(phase3Name_),
             U,
             phi
         )