Three-Loop Leading Top Mass Contributions to the rho Parameter
  We present analytical results for the leading contributions of the top quark to the electroweak rho parameter at order GF^3 Mt^6 and GF^2 Mt^4 alpha_s. The Higgs boson and the gauge bosons are taken to be massless in this limit. The correction of order GF^3 Mt^6 is found to be sizeable in comparison to the the leading two-loop GF^2 Mt^4 correction, however it is much smaller than the subleading GF^2 Mt^2 MZ^2 correction. 
J.J. van der Bij, K.G. Chetyrkin, M. Faisst, G. Jikia and T.   Seidensticker