  We compute the full three-loop coefficient functions for the structure
functions F_2 and F_L in massless perturbative QCD. The results for F_L
complete the next-to-next-to-leading order description of unpolarized
electromagnetic deep-inelastic scattering. The third-order coefficient
functions for F_2 form, at not too small values of the Bjorken variable x, the
dominant part of the next-to-next-to-next-to-leading order corrections, thus
facilitating improved determinations of the strong coupling alpha_s from
scaling violations. The three-loop corrections to F_L are larger than those for
F_2. Especially for the latter quantity the expansion in powers of alpha_s is
very stable, for photon virtualities Q^2 >> 1 GeV^2, over the full x-range
accessible to fixed-target and collider measurements.
