The drummer has (or used to, not sure if he still does) a huge bank of fx and sample triggers, and I know bellamy's guitar tech runs some of the guitar fx offstage- he used to use a massive floorboard but that's not too practical in big venues, ties you to one spot. Course, some of his guitars are effectively FX pedals too.
But yep, they do now have a 4th touring member, and it's a bit crap that he has to hide up the back. Not sure when they started with that, certainly in the earlier days it was just the 3 of them and they didn't have so many expensive toys to play with, and they still made a ridiculously complicated noise with just 3.