  We study three-dimensional $\mathcal{N}=2$ $Spin(7)$ gauge theories with
$N_S$ spinorial matters and with $N_f$ vectorial matters. The quantum Coulomb
branch on the moduli space of vacua is one- or two-dimensional depending on the
matter contents. For particular values of $(N_f,N_S)$, we find s-confinement
phases and derive exact superpotentials. The 3d dynamics of $Spin(7)$ is
connected to the 4d dynamics via KK-monopoles. Along the Higgs branch of the
$Spin(7)$ theories, we obtain 3d $\mathcal{N}=2$ $G_2$ or $SU(4)$ theories and
some of them lead to new s-confinement phases. As a check of our analysis we
compute superconformal indices for these theories.
