Well, if you feel adventurous you can try a scratch remover kit for CDROM's. I don't recommend this, but if you already have one try it (read the direction's it's not as easy as it looks). The liquid versions are better then the nasty/expensive mechanical ones.
Also make sure it's not your CDROM drive itself, my computer gets moved a lot so I keep having my CDROM drives die or cable problems. Try reading the disk on another computer (using Windows Explorer), before you seek a replacement.
The replacement fee for CD's is $13 per CD here:
http://warrantyinfo.ea.com/
Asking nicely speeds the process. This is probably worth it since the Sims 3 isn't going to be until at least 2009.
Once you get a hold of your CDROM's make your legal backup with a CDROM disk imager/emulator, so you never read from you original CDROM's again, which also makes the game load faster. DVDROM's can also be backed up and emulated.
If you have to install an earlier expansion, it's best to remove all the expansions after it before installling it. Say you have Uni, and OFB and want to install NL. You are going to have to uninstall OFB, install NL, reinstall OFB, and apply patches. Takes a long time.