NsCDE is not Window Manager, FVWM is the Window Manager. NsCDE is a hybrid lightweight DE created by scripting and a bit of programming, but largely by using FVWM (plus couple of other free software components) as a framework. This is all explained in /opt/NsCDE/share/doc/NsCDE.pdf documentation.
FVWM is a Window Manager, but due to it’s powerful and extensive nature can be (in my opinion) considered also as framework for making GUI environments.
When FVWM3 will be in some mature beta stage in which it’s final shape can be largely known, NsCDE will for sure support it and use it’s new features. With a few glitches, NsCDE can be started with fvwm3 already at present stage, but this is not stable environment for everyday use yet. FVWM 3 will be released by it’s authors when they decide it is a time.
Read fvwm3 milestones on GitHub. Main new improvement is full and dynamic RandR and multiscreen support, and major configuration paths of “global” and “per-monitor” view of desks and pages.
You can freely clone fvwm3, build, install and then participate in testing. With or without NsCDE. If you want to try NsCDE with fvwm3, I suggest you first try it with fvwm 2.6.9 to become familiar with how it functions and so, and then you can edit /opt/NsCDE/bin/nscde to change FVWM_BIN to point to fvwm3 and login to that. Then, you can test adding monitors, changing new DesktopConfiguration between global and per-monitor, test FvwmPager etc.
See issues tab on github of fvwm3, so you can see on what bugs there is currently some work done.
Here is the official call for testing from developer: