Yes, congratulations are in order for finding the 3rd waste water valve!
I've posted about the discovery of it on our RV previously. In short, I couldn't see the pull handle for the 3rd tank, as it was right on the frame member, and being black, it blended right in to the frame. We'd been out several times, and the tank was showing full, but I couldn't find the valve, so assumed it drained with the other Gray water valve. Spent about 15 minutes at the dump station looking for it, and did finally find it, but it was not easy to spot.