Dunno about the Intuos3, but my Graphire has a clear plastic sheet that goes over the tablet so you can place a document under that and trace it...never used it,and mine ended up falling off. (Was hot glued to the top) I think it also helps prtect the plastice tablet itself somewhat as mine has scratched from being stroked on, not bad though and still works great.
I would imagine thats what they are. I'm getting a Intous3 real soon...cant wait 🙂