  We consider the renormalization of the three-quark operators without
derivatives at next-to-next-to-leading order in QCD perturbation theory at the
symmetric subtraction point. This allows us to obtain conversion factors
between the $\bar{\rm MS}$ scheme and the regularization invariant symmetric
MOM (RI/SMOM, RI${}'$/SMOM) schemes. The results are presented both
analytically in $R_\xi$ gauge in terms of a set of master integrals and
numerically in Landau gauge. They can be used to reduce the errors in
determinations of baryonic distribution amplitudes in lattice QCD simulations.
