The reason for replacement of the birdcage for 3-prong was the open construction would make it more likely to catch vegatation in the suppressor's open prongs and create a muzzle obstruction.
On a related note, I had heard that their were two versions of the three prong flash suppressor. The first was a stepped (two diameters) affair, and the more known second version had the gradual taper from the bottom to the top (muzzle end). Anyone with a copy of "The Black Rifle: A Retrospective" that can confirm this?