Ok, if you are trying to watch the OVAs this is the best way to do it, just as a not a lot of people HATE OVA 3, though I quite like it. On the bright side, nearly all of Tenchi is available on Youtube from Funimation.
Anyhow:
The Best order to watch the OVAs is the following:
Tenchi Muyo! 1-13 (OVA 1 and 2) This is the original OVA series, you likely saw this on Toonami back in the day, but it was the shortest of the three series that they aired back then. This introduces Tenchi, most of the girls, and sets up the backstory, Tenchi's power, and the goddesses.
Tenchi Muyo! GXP This is a spin off set in the same universe as Tenchi Muyo and crosses over with a bunch of the characters. Major important characters that you will meet in OVA 3 are first introduced here, even though it takes place after it. This was released in Japan first, and the assumption going in to OVA 3 was that you have seen it.
Tenchi Muyo! Ryo-ohki 1-7 (OVA 3) This is the last full on Tenchi Muyo! series released in the main continuity, it finishes off the Goddess storyline, and the question of Tenchi's power. It's a bit rough, but enjoyable IMO.This series has a bunch of the characters first seen in GXP. So watch it last.
Also, be sure to watch it subbed if you can deal with that. There are a few reasons, first off, the voice actros will stay consistent throughout, but more importantly the translation of the dub is pretty bad in places and it causes a ton of confusion down the line. VERY important lines are mistranslated, and a lot of the complaints I have seen on OVA 3 reference these. Normally this wouldn't be a huge issue, but Kajishima is a bit of a crazy person, and gets very specific at times.
Once you are done with all that you can watch Tenchi Muyo! War on Geminar which is a sort of sequel to TM! that stars Tenchi's brother in another universe. Just don't go in expecting to see the main cast from TM! in anything but references and cameos.
