I would break apart each floorplan into three separate DWG files, aligning the floorplans with a common origin (align each floorplan with gridline A,1 for example). Then, Link these DWG's into Revit on the proper Level. If you know the floor to floor heights, that would be helpful to get each level spaced at the proper heights.

Then, just continue on as normal. Many things will be unavailable, obviously, since this isn't a true Revit floorplan, but at least it does get things into Revit. Attached is an example. Although, I Imported instead of Linked. I would suggest Linking, I only Imported just for ease of sharing.

Link to example: