Ok, crazy problem here. I downloaded the latest install manager for P3D v3.
There's no such thing as an install manager. Maybe you mean the Addon Manager, but that won't have any effect on the ability of a scenery installer to install.
To support P3D V3, you must download ALL your scenery installers.
Then I go to install the airports. JFK2, LAS, LAX ORD. Only JFK2 actually shows installed and active and scenery installed. All other airports pretend to install, but then when I load the sim and go to either LAS, LAX, ORD. The FSDT airports are not installed. Also in scenery library only the JFK2 shows installed.
The only possible reason is that you used an updated P3D V3-compatible installer only for JFK V2, but are still using older installers for the other sceneries. All the installers we have online now are IDENTICAL, and they ALL support P3D V3.
Then also in the drop-down add-on menu, the Add on Manager and everything else that should be associated is now not installed.
If you are saying that, after installing an scenery using an outdated scenery, you lose the Addon Manager and the Couatl menus, it's normal this would happen, especially if you use the P3D Migration Tool, which will force an installer that doesn't support P3D (maybe you tried this) to install anyway.
This won't work with our products. They supports and recognize P3D V3 natively, and because of this, you cannot use the Migration Tool, but you MUST download and install using the CURRENT version of the installers, which all supports and recognize P3D V3.