【解决方案】代码需要将添加的feature更新到LayerControl里。
SceneControl sceneControl=new SceneControl();
LayersControl layersControl=new LayersControl();
Layer3DKML layer3DKML = sceneControl.Scene.Layers[0] as Layer3DKML;//获取场景中的KML图层
GeoPlacemark placemark = new GeoPlacemark();//新建三维地标几何对象
Feature3D feature3D = new Feature3D();//新建三维要素类
feature3D.Name = "aa";
feature3D.Geometry = placemark;
layer3DKML.Features.Add(feature3D);//KML图层添加三维要素类
String kml = feature3D.ToKML();
feature3D.FromKML(kml); //从KML字符串中导入三维要素对象
layersControl.Layer3DsTree.RefreshNode(layersControl.Layer3DsTree.Layer3DsNode);
sceneControl.Scene.Refresh();