Hi Chris,
if you choose b97-3c as functional and switch on DFT-D ($disp3 bj), it will automatically add the three-body terms. But it does of course no harm if you include it explicitly by adding -abc to the $disp3 option.
So setting the functional name and adding $disp3 bj is sufficient.
TmoleX does, by the way, also allows one to use B97-3c, just make sure to switch from DFT to DFT+Dispersion in the method panel.
A remark: While you are allowed to use any basis set you like, B97-3c is designed for the mTZVP set (more or less def-TZVP, but with some modifications for some elements like a different polarization function).
Regards,
Uwe
