The ERR3 means it's jammed. It's a mechanical item, and I guess it's not just that reliable. Yes, it should be able to handle 6 discs with no problem, and mine used to be fine with 6 discs in it, but after a while it got ERR3. As I said, in theory you could take it apart, free the jam and put it back together and it would probably be fine again (till the next time it jammed). Trouble is, the bottom loaders are pretty hard to take apart so IMHO would be even harder to put back together.
Think of it as a blessing in disguise though. I was a bit annoyed when my head unit went ERR3, but in the end I was pleased, because my new head unit is sooooo much better ! :D