A 3rd loader will only appear if you configured the airplane to have a main cargo door on the left.
The configurations supplied with GSX for the PMDG 777 has the main cargo door enabled only for the Freighter version, with the passenger versions you won't see 3 loaders, as you can see in the attached screenshots, which shows 2 loaders with the passenger version, and 3 loaders with the Freighter version so, if you are seeing anything different, it means there must be a problem with the custom configuration for the PMDG 777 you made/downloaded, and should be fixed by disabling the main cargo door, as in the configuration we ship with GSX as default.
We don't supply a pre-made configuration for the QW787 so, I cannot comment on that but, the solution should be the same: disabling the main cargo door if it's not required.
Of course, I assume you have your GSX fully updated to the most current version, which means you applied the latest FSDT Live Update.