Is it possible with a single keystroke to autofit 3 floating windows to my MacBookPro screen?
Will MultiView handle what you want to do? It's the easiest way to view multiple resources side-by-side.
I remember Mark Ward mentioning third-party apps in this Logos blog post.
I don't think SizeUp does thirds, but BetterSnapTool can dock windows to left/middle/right thirds.
I don't know if there's a single-keystroke solution (within or outside Logos) but Mark or the app developers might know.
There is a window management tool called "Magent" click HERE to see on Apple Apps store. It works with three floating windows. All you do is drag each window to the bottom of your screen in the area of the 1/3 you want it. See attached screenshot:
