Hi@hyezero,

The workspace which you linked to simply takes the geometry in the 3DS file and merges everything into a single Building feature which sounds like is not what you want.

CityGML can be a bit of a challenge to write out to so you may need to do a bit of learning/testing before you can get the output you want - You will need to learn this a bit:https://docs.safe.com/fme/html/FME_Desktop_Documentation/FME_ReadersWriters/citygml/Tutorial/writing_citygml_from_fme.htm

It sounds like you will need to do four things:

1.Read in the 3DS (easy part) - Just add 3DS Reader
2.Separate features into surfaces - (Something like a Deaggregator)
3.Create some logic to determing which is Wall, Roof and Ground.If you have innter surfaces you will want to somehow remove those.
4.Write out the CityGML, can be a bit tricky

Here is a link which contains a few templates which will help you at least with step four.Steps 2 and 3 could be tricky and will be probably specific to your data:https://www.virtualcitysystems.de/en/news/480-3d-solution-templates-now-available-in-the-fme-hub

Happy Converting!