  We study the scattering amplitudes of mass-deformed Chern-Simons theories and
Yang-Mills-Chern-Simons theories with N=2 supersymmetry in three dimensions. In
particular, we derive the on-shell supersymmetry algebras which underlie the
scattering matrices of these theories. We then compute various 3 and 4-point
on-shell tree-level amplitudes in these theories. For the mass-deformed
Chern-Simons theory, odd-point amplitudes vanish and we find that all of the
4-point amplitudes can be encoded elegantly in superamplitudes. For the
Yang-Mills-Chern-Simons theory, we obtain all of the 4-point tree-level
amplitudes using a combination of perturbative techniques and algebraic
constraints and we comment on difficulties related to computing amplitudes with
external gauge fields using Feynman diagrams. Finally, we propose a BCFW
recursion relation for mass-deformed theories in three dimensions and discuss
the applicability of this proposal to mass-deformed N=2 theories.
