Sounds like you may have an open circuit on solenoid 3 (i.e. it’s disconnected). Solenoid 3’s only function in life is to pressurise the opposite face of the reverse gear valve (RG-V) spool - thereby preventing engagement of the C & F clutches - in the event of the gear lever being moved into reverse while the vehicle is travelling forward at speed. Consequently it may never actually be required throughout the whole lifetime of the vehicle (unless the driver has a habit of doing this!)
However, it is also activated in Neutral, which is presumably why it is being flagged by your controller.
The easiest way to test it (without having to take the sump off) is to check the resistance at the 16-pin connector.
If you measure between pins 4 & 16 you should get a reading of between 26 and 30 Ohms.
I have loads of (used) spares if you need one.
Phil