The realization of three-dimensionally confined GaAs/AlGaAs structures on GaAs (111)B and GaAs (001) substrates via one step in situ molecular beam epitaxy is reported. Growth is carried out on nonplanar patterned substrates with crystallographically equivalent mesa top edges. Equivalent side facets evolve during growth and completely surround the mesa top. Adatom migration from these facets to the mesa top result in shrinkage of the mesa top area leading to mesa pinch-off. Scanning and transmission electron microscopy provide evidence for the realization of structures with lateral linear dimensions less than or similar 500 angstrom. Cathodoluminescence images from the (111)B structures attest to their high optical quality.