  Using $\mathcal N=8$ supergravity as a theoretical laboratory, we extract the
3PM gravitational eikonal for two colliding massive scalars from the classical
limit of the corresponding elastic two-loop amplitude. We employ the eikonal
phase to obtain the physical deflection angle and to show how its
non-relativistic (NR) and ultra-relativistic (UR) regimes are smoothly
connected. Such a smooth interpolation rests on keeping contributions to the
loop integrals originating from the full soft region, rather than restricting
it to its potential sub-region. This task is efficiently carried out by using
the method of differential equations with complete near-static boundary
conditions. In contrast to the potential-region result, the physical deflection
angle includes radiation-reaction contributions that are essential for
recovering the finite and universal UR limit implied by general analyticity and
crossing arguments. We finally discuss the real emission of massless states,
which accounts for the imaginary part of the 3PM eikonal and for the
dissipation of energy-momentum. Adopting a direct approach based on unitarity
and on the classical limit of the inelastic tree-level amplitude, we are able
to treat $\mathcal N=8$ and General Relativity on the same footing, and to
complete the conservative 3PM eikonal in Einstein's gravity by the addition of
the radiation-reaction contribution. We also show how this approach can be used
to compute waveforms, as well as the differential and integrated spectra, for
the different radiated massless fields.
