What floor plan do you have? If I was a guessing man it is a 2861...that is only for the outside kitchen drain.
The FR3 does not have an outside kitchen...and would probably not need a 3rd drain since it sits higher. But based on slope, etc, there was no way to get the outside kitchen sink to drain into the normal gray water tank so it had to have its own drain. It will see very little water and it is all pretty clean gray anyway.
You are in a motorhome sub-forum, but stating that it is a motorhome and the floor plan will help. Others without the same floor plan will try and help, but your best answer will come from the factory or others with the same model.
PLEASE do not send questions via Private Message, my inbox fills up too quickly. Post questions in the forum as a new thread so that other users can benefit from the answers. Use Private Messages for sensitive information only (VIN, Contact info or things you do not want the public to see).