This sample shows the visual difference between flat symbol layers and volumetric symbol layers when visualizing a 2D FeatureLayer in 3D, using IconSymbol3DLayer(flat) or ObjectSymbol3DLayer (volumetric). To visualize point symbols in 3D, you must use PointSymbol3D as the container of either an IconSymbol3DLayer or an ObjectSymbol3DLayer.
Notice how the symbols are billboarded on the surface of earth. Compare this sample with ElevationInfo to understand different choices of visualizing 2D point features in 3D.
To learn more about symbol layers and the differences between flat and volumetric symbol layers, read the documentation on Symbol3DLayer. Also see the Labeling guide page for more information and known limitations.