Please refer to the following link in the Ni developer zone for plotting multiple 3D plots on a single 3d graph.
http://zone.ni.com/devzone/explprog.nsf/6c163603265406328625682a006ed37d/a42072a6b4ad885b8625674e00072a4e?OpenDocument
Regards,
Cyril Bouton
Applications Engineer
National Instruments
Cyril Bouton
Active LabVIEW Developper