The present article investigates Sp(3) structures on 14-dimensional
Riemannian manifolds, a continuation of the recent study of manifolds modeled
on rank two symmetric spaces (here: SU(6)/Sp(3)). We derive topological
criteria for the existence of such a structure and construct large families of
homogeneous examples. As a by-product, we prove a general uniqueness criterion
for characteristic connections of G structures and that the notions of
biinvariant, canonical, and characteristic connections coincide on Lie groups
with biinvariant metric.Comment: 24 page