  We compute the three graviton amplitude in the
Banks-Fischler-Shenker-Susskind matrix model for M-theory. Even though the
three point amplitude is determined by super Poincare invariance in eleven
dimensional M-theory, it requires a non-trivial computation in the matrix
model. We consider a configuration where all three gravitons carry non-zero
longitudinal momentum. To simplify the problem, we compactify one additional
dimension and relate the amplitude to a supersymmetric index computation. We
find agreement with the expected answer even at finite values of $N$.
