I attach an Marshall 7" external monitor to 5Dmk3
I have image when i record video.
However, when i switch to play mode, I couldn't see anything

Is it normal? anyone know how I can see video playback from 5D mk III to external monitor

I have mine set to HDMI output + LCD No mirroring, and as far as i remember i can view playback.

If you have it set on mirroring then you cant view playback on ext screen, but if you put it in no mirroring, then you can view on ext screen.