I discovered this one accidentally since I really did want to open three projects at the same time after opening Xcode.
It turns out that the “Open Project…” dialog in Xcode supports multi-select! So you can Shift-Down/Up to select more than one project, hit Return and all three projects will be opened simultaneously!
I pretty much stumbled on this one when I did an “I wonder if this works” kind of thing at that dialog, and sure enough, it opened all three.
Score!!!