3D perspective mode using xy package

Second approach

With the extension Rotate and Scale extension by Ross Moore you can also achieve a similar output. Therefore, load the option xyoption{rotate} and add the argument [@!<number>] . For further details, see p. 29 of Reference manual

documentclass[a4paper,12pt]{article}
usepackage{amssymb}
usepackage[all,cmtip]{xy}
xyoption{rotate}


begin{document}
    defangle{50}
    xymatrix@!0{
        & *+[@!angle]{alpha} ar@{-}[rr]ar@{-}'[d][dd]
        & & *+[@!angle]{beta D} ar@{-}[dd]
        \
        gamma ar@{-}[ur]ar@{-}[rr]ar@{-}[dd]
        & & delta A ar@{-}[ur]ar@{-}[dd]
        \
        & *+=[@!angle]{ax} ar@{-}'[r][rr]
        & & *+=[@!angle]{bxximu}
        \
        Gamma_r(f(x,y)) ar@{-}[rr]ar@{-}[ur]
        & & lambda P ar@{-}[ur]
    }
end{document}

First approach

The answer of David Carlisle to Rotating labels in xypic shows you a possible solution, that is, using for example the rotatebox command of the graphicx package.

documentclass[a4paper,12pt]{article}
usepackage{amssymb}
usepackage{graphicx}
usepackage[all,cmtip]{xy}


begin{document}
defangle{50}
xymatrix@!0{
& rotatebox[origin=c]{angle}{$alpha$} ar@{-}[rr]ar@{-}'[d][dd]
& & rotatebox[origin=c]{angle}{$beta D$} ar@{-}[dd]
\
gamma ar@{-}[ur]ar@{-}[rr]ar@{-}[dd]
& & delta A ar@{-}[ur]ar@{-}[dd]
\
& rotatebox[origin=c]{angle}{ax} ar@{-}'[r][rr]
& & rotatebox[origin=c]{angle}{$bxximu$}
\
Gamma_r(f(x,y)) ar@{-}[rr]ar@{-}[ur]
& & lambda P ar@{-}[ur]
}
end{document}