I have 3-display setup that were working properly with Parallels under OS X 10.8, but when I've updated to OS X 10.9 (Mavericks) GM build, Full Screen mode is stopped working. Guest os (windows) detects all 3 host monitors, and even when I do print screen, the resulting image contains all 3 desktops, but these desktops aren't shown anywhere in host os (OS X 10.9 - Mavericks)
P.S.
When you wiggle around displays in guest os you might get Parallels take advantage of second display, but when you try to swipe any full-screen app on any of 3 displays, it makes guest os go crazy, it starts thinking that monitor configuration is changed, the screen repeatedly goes black, resolution of displays inside guest os may be changed, and after around 10 iterations of previously described action, the displays inside guest os become unavailable. You may get your displays back when you reinstall parallel tools
P.S.
When you wiggle around displays in guest os you might get Parallels take advantage of second display, but when you try to swipe any full-screen app on any of 3 displays, it makes guest os go crazy, it starts thinking that monitor configuration is changed, the screen repeatedly goes black, resolution of displays inside guest os may be changed, and after around 10 iterations of previously described action, the displays inside guest os become unavailable. You may get your displays back when you reinstall parallel tools