Tried to update iPad 3 (GSM version) to IOS7; after 10 minutes or so, the 'plug into iTunes' prompt appeared. I plugged in and attempted to restore and got an 'unknown error(-1)'. Repeated (a dozen times) and kept getting the same message. Followed the steps in this post and now I am getting an 'unknown error(3)'. Can't find a reference to this error. Any thoughts?
Error3 is usually a synchronisation error, and nothing to do with a restore.
Since you are trying to install IOS7 are you SURE you are using the latest versions of everything? You must be on the latest iTunes to install 7 flawlessly. To be honest, you really shouldn't need to follow this tutorial at all. I plugged my fully jailbroken iPad4 into iTunes and simply hit restore and IOS7 was installed from start to finish without doing anything special at all.