@OP

You have a 120gb Slim, right?

The following is an actual workaround, so don't laugh.

Do this: Eject the game disc. Grab it with one hand. With your other hand tilt your PS3 up (around 45º) lifting it from the front. Insert the game disc and wait a couple of seconds until it reads, then you can put it down.

Worked for me 100% of the times.

If this doesn't do, you can try googling for "won't read Uncharted 3" since many people had the same issue with that.

Hope it helped.