  We consider 3d N=1 M-theory compactifications on Calabi-Yau fourfolds, and
the effective 3d theory of light modes obtained by reduction from eleven
dimensions. We study in detail the mass spectrum at the vacuum and, by
decoupling the massive multiplets, we derive the effective 3d N=1 theory in the
large-volume limit up to quartic fermion terms. We show that in general it is
an ungauged N=1 supergravity of the form expected from 3d supersymmetry. In
particular the massless bosonic fields consist of the volume modulus and the
axions originating from the eleven-dimensional three-form, while the
moduli-space metric is locally isometric to hyperbolic space. We consider the
F-theory interpretation of the 3d N=1 M-theory vacua in the light of the
F-theory effective action approach. We show that these vacua generally have
F-theory duals with circle fluxes, thus breaking 4d Poincar\'e invariance.
