public bool IsFlattened {get; set;}

{
    //The layer in the 2D group to move to the 3D Group in a Local Scene
    //Get the layer's definition
    //Note: needs to be called on the MCT
    var lyrDefn = featureLayer.GetDefinition() as CIMBasicFeatureLayer;
    //setting this property moves the layer to 3D group in a scene
    lyrDefn.IsFlattened = false;
    //Set the definition back to the layer
    featureLayer.SetDefinition(lyrDefn);
}