[User 309201]

Is 3d frame packed supported on these builds?

Yes. Make sure you've enabled "Enable Full HD HDMI modes for stereoscopic 3D" and "Adjust display refresh rate to match video".

Despite having done both of these things, 3D .mkv files aren't playing in 3D on my RPi running LibreELEC 8.90.003 (Leia alpha 3) - is that a version where you'd expect this to be working, or is it only enabled in more recent nightly builds? (Is there something else I should check?)