public bool IsFlattened {get; set;}

//The layer in the 2D group to move to the 3D Group in a Local Scene
var layer = MapView.Active.Map.GetLayersAsFlattenedList().OfType<FeatureLayer>().FirstOrDefault();
QueuedTask.Run(() =>
{
  //Get the layer's definition
  var lyrDefn = layer.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
  layer.SetDefinition(lyrDefn);
});