Using the .NET API you can interact with C3D objects in a different database. I was never able to do the same using the COM API. However, even though you CAN do it, doesn't mean it's always a good idea.
More specifically, editing a C3D dwg while it is not open in the Editor, and subsequently saving the dwg, often leads to unwanted results when you have Line/Curve Tags and Tables....the tags will get wiped out, almost every time, especially if they are labeling Xref and/or Dref objects.