I want to type contraction of 3*3*3 matrices (the triple dot product symbol), how to write?

My welcome to the TeX.SE... I have added a fast answer....with the suggestion of the very nice user @egreg.

 documentclass[a4paper,12pt]{article}
    usepackage{amsmath,amssymb}
    usepackage{newtxtext,newtxmath}
    begin{document}
    $21(boldsymbol{mathrm{n}}_{ab}^{(3)}mathbin{therefore}boldsymbol{Omega}_{a})(boldsymbol{mu}_bcdotboldsymbol{mathrm{n}}_{ba})$.
    end{document}