How to set up a 3-screen video installation to sync correctly?
The Matrox graphics card is probably the easiest and most versatile approach. I use the TripleHead2Go, which as you'd assume sends a signal(s) to up to three monitors/projectors.

Multiple computers will never stay in sync. I found this out the hard way the night of an event.

In terms of software, if you're on OS X check out VDMX. There's a fully functional demo that you can grab. Setting it up for your needs would be fairly trivial after a quick search for the right approach.

PS. I would have just commented on the other answers, but I'm new here and don't have the credentials.