It's a considerable time since I upgraded, but I went straight from B3 to B5 when I did it. You will find you can open saved B3 scenes. When you click on "open", at the bottom it says B5 files, but that has other options in a drop down list, including as far back as B2 files. I seem to remember that the best way to transfer skies was in scene files. I know there were some things I couldn't transfer, have a strange feeling that it was the bryce objects which had a different suffix, not obp, but don't take that as gospel.

The greatest part of wisdom is learning to develop  the ineffable genius of extracting the "neither here nor there" out of any situation...."