  The leading eikonal S-matrix for three graviton scattering in d=11
supergravity and Matrix Theory are shown to precisely agree. The result unifies
the source-probe plus recoil approach of Okawa and Yoneya and relaxes the
restriction imposed by those authors that all D-particle impact parameters and
velocities are mutually perpendicular. Furthermore, the unified S-matrix
approach facilitates a clean-cut study of M-theoretic R^4 curvature corrections
to the low energy supergravity effective action. In particular, the leading R^4
correction to the three graviton S-matrix is computed and compared to the
corresponding next to leading order two loop U(3) amplitude in Matrix Theory.
We find a clear disagreement of the two resulting tensor structures.
