Can I install an older version of Libreelec on Pi 400 that will allow me to play 3d videos? Thanks.
It's a software issue, not hardware. You might be able to run the older LE9.2 image (which supports 3D). I have fuzzy memeory there may be newer iterations of the 400 that need firmware newer than what's shipped in the 9.2 image. I could be confusing it with RPi Zero2 though? (easiest way is to image an SD card and test).