This will never be implemented in CoreELEC as it would require much changes and may introduce other side effects. Also Home Cinema 3D is official confirmed as “dead”.
It’s also bad to me as I own a 3D capable TV myself. And I own 1 3D BluRay 
I pushed last status for you on my private repo. Feel free to use it and rework it as it’s surely outdated.
https://github.com/Portisch/CoreELEC/commits/coreelec-9.2-3d
Please read also this thread: