I wish I had an answer to your question, because I'm having the same problem.
I was able to recover some of them by checking different catalogs. Have you checked to see if there are any other catalogs in your Lightroom? You can access them by clicking on File and then on Open Recent, if you don't know that already.
I was also able to recover a few after discovering that I was saving photos to my computer's hard drive instead of the external hard drive I thought I was saving them to. Could that be your problem?
If not, then we both need help! The difference is that I have LR4, not 3.
I will be checking those options now. I found SOME...just not the ones i need to edit for my clients, of course. :( Kristie Baer
Offline or missing can be fixed by following these instructions: for LR5: Adobe Lightroom - Find moved or missing files and folders, or for LR 2 through LR 4: Adobe Lightroom - Find moved or missing files and folders