To me, it's not about number of screens, but the total amount of available screen space (both pixels and inches).
I regularly use one large screen (26" widescreen) for most of my work. I find this screen large enough to do side-by-side work when necessary. If I needed to have 3 items open at once (code/documentation/google search?), I would probably find it easier if I had more space than I do.
That said, when I'm really in the groove, I don't want anything but what I'm working on visible on my main screen. If I need a reference document open to glance at once in a while, I do find it better to have it on a separate screen, where it doesn't interfere with my concentration, but still lets me see both at once if I need to.