I am not aware of a third relay on my car, but I will take a quick look later today. I believe the relays may be wired in such a way that if the driver side fails, both seats are disabled. If the passenger side fails, only that side is disabled.
Some people have diagnosed that by swapping the relays, but I never needed to go that far. If you take the driver side relay out and to a part store, you should be able to get a new one for $10-20, but just as likely, it may be a loose wire or connector.
Unfortunately the electrical system on this car was not designed to last. The connectors are good quality, but the plastic gets fragile after a few years and the latches snap off. The electrical tape on the wires does not last more than 6-8 years, then the wires can get loose, catch on something, and break or short.
The seats have three electric motors each (I think). If a motor fails, or the wiring to the motor fails, then one axis of motion stops working. The switches that control the seats can get dirty and stop working also.