  The maximal supergravity theory in three dimensions, which has local SO(16)
and rigid $E_8$ symmetries, is discussed in a superspace setting starting from
an off-shell superconformal structure. The on-shell theory is obtained by
imposing further constraints. It is essentially a non-linear sigma model that
induces a Poincar\'e supergeometry that is described in detail. The possible
$p$-form field strengths, for $p=2,3,4$, are explicitly constructed using
supersymmetry and $E_8$. The gauged theory is also discussed.
