We report three-dimensional micromagnetic structures obtained for magnetite cubes with $langle$100$rangle$ edges in the size range 0.03--0.3 ¿m around the single-domain threshold size. We employed the Metropolis algorithm to find equilibrium micromagnetic structures starting from uniformly magnetized initial states. Directions of initial magnetizations were randomly set so as to mimic a randomly oriented assemblage. Small grains (<0.07 ¿m) exhibit single-domain equilibrium structures parallel to a magnetic easy axis $langle$111$rangle$ irrespective of initial magnetization directions. Vortex structures emerge for small pseudosingle-domain grains above 0.07 ¿m. Axes of the vortices are parallel to $langle$100$rangle$ in the threshold size range but become randomly oriented with increasing grain size. Neither lamellar domain structures nor multiple vortex structures were found for grains smaller than 0.3 ¿m. Reduced saturation remanent magnetization decreased down to ~0.1 with increasing grain size over a relatively narrow grain size range (0.07--0.15 ¿m). The grain size variation of reduced magnetization depends on microscopic magnetization structures inside a single grain and the degree of randomness of grain magnetization directions in an assemblage.