how old is your xbox?
I've read several threads about sammy (and my primary xbox is sammy too) and xgd3 issues.
Your issue might be that the laser is going bad. You have to remember that xgd3 games are more laser intensive and that will put strain on the laser. Old lasers may not be able to read it very well.
If you don't get a working solution from these guys, I would suggest looking into buying a new laser or if you don't want to deal with lasers buy a whole new drive. Spark (a very regular member on here) had a thread where the xgd3 games were not being read from his sammy and he replaced the laser and presto everything worked.
Hope this helps!
---------- Post added at 15:27 ---------- Previous post was at 15:27 ----------
Was the laser very noisy when reading discs before it stopped working?
I had one like this and I tried to Pot-Tweak it which worked sporadically but once I installed a new laser it was perfect and a lot quieter
Damnit! Seconds before I posted! You're killing me here...:wink: