ERR 3 (when a CD gets jammed in the changer) is probably the most common problem along with alloy wheel corrision. It SHOULD be covered under warranty, in th UK the warranty for the stereo is 5 years.
There are a number of things you can try to fix the problem if you do a search on ERR 3.
There are a couple of french members who post on here, maybe they could tell you where to go if you cant fix it yourself.