This paper presents the architecture for an extensible toolkit used in construction and rapid prototyping of three dimensional interfaces, interactive illustrations, and three dimensional widgets. The toolkit provides methods for the direct manipulation of 3D primitives which can be linked together through a visual programming language to create complex constrained behavior. Features of the toolkit include the ability to visually build, encapsulate, and parameterize complex models, and impose limits on the models. The toolkit's constraint resolution technique is based on a dynamic object model similar to those in prototype delegation object systems. The toolkit has been used to rapidly prototype tools for mechanical modelling, scientific visualization, construct 3D widgets, and build mathematical illustrations.