Originally Posted by Road Fan

Its all a matter of whether the largest rear sprocket can be handled by the rear derailleur, and the total change in the number of links that must be wrapped by the derailleur cage are within its design capability. The rear derailleur only cares about those two parameters, not how many rings are on the chainset. The sizes of the rings, yes, but the number of the rings no. Master how all of that works and you, too, will be a master gear finagler! I'm pretty sure Sheldon Brown covers all of that, it's not so obscure or hard to find.