I'm not sure where that other ground wire goes once it integrates with the wire harness, but the third ground point is actually on the back of the engine block to the firewall. Here is a picture looking from the passenger's side at the back of the engine block. You can see the bolt, the original (small) ground wire, and my 2 gauge ground wire covered with black loom. I added the 2 gauge ground and kept the factory ground wire intact.
I ran the new engine block ground cable to the passenger's fender wall and attached it with a ground terminal screwed into an existing, unused bolt hole.