If there is only one plots in your 3D graph, you can double-click on the "1" layer icon on the top-left side. Then the "Layer Contents" dialog will pop-up, you can select the plot in the left panel and add it to the right panel. Then the graph will have two plots, you go on follow the step 3 in the following page to custom the graph. https://www.originlab.com/doc/Tutorials/3D-Stack-Surface