FYI, I maintain a Gentoo 64-bit image that boots on the RPi3 B and B+:
(Full download and usage instructions on the above page)
Boots straight to an Xfce4 desktop, supports WiFi, Bluetooth, sound, VC4 GL rendering.
Comes complete with a reasonable set of pre-compiled apps (libreoffice, firefox quantum, vlc, gimp etc) so you don't need to compile or configure anything just to try it out.
It is also available for download via PINN
, should you prefer that route (named "Gentoo64" there).
For more details (and a screenshot!) see e.g. this thread
(also on this thread
, near the end, are some posts from people who've tried it).