if you want each screen run on separate video card here's a mobo with 3 PCI ports
http://www.ewiz.com/detail.php?name=...fa5a86ee08ea7b
this mobo is about year old or so no SATA3 though
then there are video cards that support more than two monitors. if you do serious rendering - better to dedicate separate powerful video card specifically just for that. and have dual screens running on another card for less intense graphic processing, most modern mobos have 2PCI slots and support lots of gigs of ram. just find a right combo for your specific needs.