I’m wondering if it’s the quality of your switching units, as I use a VGA KVM switch and the picture quality is perfectly fine for me. Have you tried different switches? For my own particular needs, I use this 4 port KVM switch… http://www.belkin.com/us/p/P-F1DS104J/
I also use (2) PC’s & (3) 24" screens, though use a different configuration than you. (2) of my screens are connected ‘directly’ to one PC’s dual video card (not through my KVM), while my 3rd screen is on my 2nd PC via the KVM switch… in anticipation of additional PC’s, which will also be going through the KVM, sharing that same 3rd screen. My keyboard & mouse is going through the KVM’s USB port to both PC’s (and future PC’s). Of course I may change my mind on my configuration as needed.