I assume you mean pre-produced, not live. Which is a whole different animal.
Pre-produced video that lets people or objects or type move across three screens seamlessly, is often done by making one very large, super-wide composite project in AfterEffects, the size of all three screens. You then feed that composite program out of a computer had drive thru a special 3-ouput video card that splits the giant, oversized frame into the three segments, all in synch. One of the cheaper ways this is done is using the matrox triple head 2 go breakout unit, playing from a PC. To get more and perhaps better advice, ask this question also in the Live and Stage forum where the experts who do this every week hang out.
Units that can mix, scale, and position cameras and sources to feed three screens live are complex and expensive, and are usually rented from a company with experience that specializes in such events.