  We show that three-dimensional supergravity amplitudes can be obtained as
double copies of either three-algebra super-Chern-Simons matter theory or that
of two-algebra super-Yang-Mills theory, when either theory is organized to
display the color-kinematics duality. We prove that only helicity-conserving
four-dimensional gravity amplitudes have nonvanishing descendants when reduced
to three dimensions; implying the vanishing of odd-multiplicity S-matrix
elements, in agreement with Chern-Simons matter theory. We explicitly verify
the double-copy correspondence at four and six points for N=12,10,8
supergravity theories and discuss its validity for all multiplicity.
