According to examples, a three-dimensional (3D) fabrication system may include fabrication components and a controller. The controller may control the fabrication components to fabricate parts of a 3D object in a build envelope, the parts to be assembled together to form the object, and in which the parts are fabricated at locations corresponding to relative positions of the parts with respect to each other in the assembled object. The controller may also control the fabrication components to fabricate a cage around the parts in the build envelope.