Sorry to welcome you to a very common and well documented problem - You will find lots of info using the SEARCH feature on the ERR3 problem.

Tone posted a CD changer reset procedure HERE

Hopefully it will allow you to at least get your CDs back. As far as getting the CD changer back to work again, I guess ERR3 is the beginning of the end for it, as you will see in all the previous posts on the subject.