Can Mavericks Spaces support 3 windows spanned across 2 displays?
Yes.

In Mavericks, each space (or full-screen application) can be assigned to any screen, and the 4-finger-swipe gesture will slide between spaces/apps on that screen only.

You can see this quite clearly if you open up Mission Control with multiple monitors; each monitor's spaces/apps are shown across the top of said monitor.

So, for example, you could put Desktop 1 and Desktop 2 on your left monitor, Desktop 3 on your right monitor. Swiping on your left monitor would change between Desktops 1 &amp; 2, while the right monitor would remain on Desktop 3.