13 CDs is less than 10Gb of data, so what I would suggest is that you copy the content of each CD to a folder on a hard disk This has two benefits - the first is that it removes the need for disk swapping, and the second is that it verifies that the data on each CD is actually readable.
From the error you are getting, it would seem that CD 3 is possibly unreadable, at least on the optical drive you are using. If this is the case, and you are unable to read this CD on any other optical drive, then you have to accept that this backup set is lost.