The GML support is well able to handle also the import of CityGML files. For more information please see the GML Data Import
If the attribute srsDimension is present for a geometry, this information is evaluated. In other cases, the dimension of a geometry is tried to be determined automatically. This works very reliable for LinearRings or geometries based on them.
Currently there is no dedicated support for viewing the 3D geometries, though you can display the 2D projection of 3D geometries in the map.