First, see this article: http://forums.adobe.com/message/3392837#3392837
Next, if you will Share/Publish/Export from each Project, you can then Import the resultant output files into it, and edit them together.
Hi Bill, Thanks very much again. Yes, I've just read the article. (I know I said I was going to bed hours ago ...)
But I just thought why don't I go to Organizer, Shift-click all the .prel files in my 3-minute project, and click Edit with PE Editor. I did, and they all opened, and I've Saved-As with a new filename.
That's really all I wanted to do. I don't know why it didn't occur to me before. Perhaps I should get less sleep.
I must come back to this tomorrow (ie later)