old Radeon HD 3000 work on the latest LibreELEC version as well as they do.
Incidentally, I just tested an HEVC file 3840x1600 24 FPS and it works fine! Unfortunately, bigger ones (e.g. 3840x2064 or 3840x1920) stutter noticeably.
according to this:
Supports HDMI with max. resolution up to 1920 x 1200 (1080P) (@60Hz)
Supports DVI-D compliant with HDCP with max. resolution up to 2560 x 1600 (@60Hz)
Supports RGB with max. resolution of 2560 x 1440 (@75Hz)
I'm overask why you can watch with 3840x1600 24 FPS !