  In the framework of the quasipotential method we calculate the contributions
of the kind \alpha^6\ln\alpha, alpha^6 of three-photon exchange diagrams to the
energy spectrum of muonium $n^3S_1$ state in the leading order on $m_e/m_\mu$.
Analytical expression of obtained correction for arbitrary principal quantum
number is equal to (Z\alpha)^6(m_e/m_\mu)m_e(6\ln(2)-35/48)/n^3.
