You need to have at least two matrix objects in the same matrixsheet. When you select any matrix object, all the matrix objects will be included in the same layer of the graph.
Activate the matrixsheet and select a matrix object.
From the menu, choose Plot > 3D : Multiple Colormap Surfaces.
Click the Multiple Color Map Surfaces button on the 3D and Contour Graphs toolbar.
(Installed to the Origin program folder).