>>2519872The first manga, known as No Need For Tenchi in the US, takes place right after the first 6 episodes of the first OVA. I believe it has been retconned out of existence, even though later issues make references to happenings in the second half of the first OVA.
The second manga, The All New Tenchi Muyo, I don't believe fits anywhere, either, but I could be wrong.