  We review the construction of almost contact metric (three-) structures,
abbreviated ACM(3)S, on manifolds with a $G_2$ structure. These are of interest
for certain supersymmetric configurations in string and M-theory. We compute
the torsion of the $SU(3)$ structure associated to an ACMS and apply these
computations to heterotic $G_2$ systems and supersymmetry enhancement. We
initiate the study of the space of ACM3Ss, which is an infinite dimensional
space with a local product structure and interesting topological features.
Tantalising links between ACM3Ss and associative and coassociative submanifolds
are observed.
