  We calculate the thermodynamic functions of a quark-gluon plasma for general
N_c and N_f to three-loop order using hard-thermal-loop perturbation theory. At
this order, all the ultraviolet divergences can be absorbed into
renormalizations of the vacuum, the HTL mass parameters, and the strong
coupling constant.We show that at three loops, the results for the pressure and
trace anomaly are in very good agreement with recent lattice data down to
temperatures T~2T_c.
