You can import 3D models (3Ds) within the composition by clicking the small icon in the upper right corner of the 3D panel then select from the drop down "add objects" and "file" ... It open the usually wizard to search for the folder and file in your computer.
Thank you, Gaetano.
That's helpful. It appears this does NOT replace an existing model, but this answers my question of how to bring a model into an existing space.
I notice if I have a have a model folder selected when I use this "add object" method, the new object folder is nested inside the existing object folder. PS does not seem to like this because I cannot drag it out of the folder or delete it. I guess I need to select the parent "scene" to avoid this issue.
I was hoping I could simply "update" a model with a newer version. Is that possible?