Three-loop large-$N_c$ virtual corrections to $gg\to HH$ in the forward limit

We compute the three-loop form factors for $gg\to HH$ in the limit of vanishing transverse momentum of the Higgs boson which provides a reasonable approximation of the cross section. In our calculations we adopt the large-$N_c$ limit, which already includes non-trivial non-planar Feynman diagrams. We discuss the results for top quark masses in the pole and $\overline{\rm MS}$ schemes and show that the scheme dependence is significantly reduced at next-to-next-to-leading order.