If you want to sue AF or initiate a dispute with your card issuer before even requesting a refund of the fare difference, that is up to you.
You then face two problems:
1. The ticket may be in error and the fare charged is the Y fare and thus the refund would be EUR 0. Hopefully your time is worth more than that.
2. G3 may simply show an add/collect on the ticket. Nobody at POA will know what you are talking about and you will simply face repaying or at worst not having a ticket.
It seems to me that your first step is to determine what the fare difference is and what you paid.