When KeepExtremes sampling function is used, EmptyPointBehavior.Gap and EmptyPointBehavior.Drop settings are ignored and points appear as if EmptyPointBehavior.Zero is used.
RadChart should support 3D Surface Series Type. This feature is now available as part of the ChartView control since R3 2016 Official release. Refer to the link below for more information. http://docs.telerik.com/devtools/wpf/controls/radchartview3d/series/surfaceseries3d.html