I lost ED-D in Vault 3 fighting Fiends and Motor-Runner, HOW do I found him again?
Does he go back to his HOME town, if so where is it?

ED-D tends to get lost, what probably happened is he got lost when you were swimming through one of the flooded passages and he glitched out trying to follow you. In my second playthrough the game I always told ED-E to wait before I went swimming and managed to avoid him getting lost.

The latest patch for PC[which is coming later for the consoles], does two things for companions I hear
Companions now show up as waypoints on the map
Companions will always fast travel with you, unless told to wait or sent away

So in theory when the patch arrives for 360 and ps3, you should be able to get ED-E back.

Are you on console or PC? If you are playing on Xbow or PS3 sorry man I can't help you. But if you are on PC there is a simple console command you can do depending on the type of ED-E you have. Did you finish ED-E's companion quest?

Iv haven't had this problem with him yet i might have just got lucky?

XBox 360, but thanks for the reply.

ED-E tends to get stuck in doorways and stuff, thanks to his floatiness. However, from time to time (and usually in vaults), both Boone or ED-E will go missing. Even though I have them on passive, sometimes we'll get to a spot where an enemy is on the other side of the wall. Perhaps they sense this as a close threat, so they'll both go running around the maze of halls and doors till they can get to the enemy (doesn't always work).

Now, usually, my solution is to simply move to another part of the vault, ie another floor. That seems to bring them back sometimes. Otherwise, before I leave a building, if I'm missing my companions, I'll just wait in a safe spot (not just hit the wait function, but wait RL). Only do this if you're patient and have something else to do. Sometimes, in a few RL minutes, they'll catch up with you.

I know the PC has tricks to bring back companions, and I assume that would work with missing companions. But I don't know if there's anything like that for consoles.

I think you may be screwed on 360. My friend lost him in Vault 34 and we tried every trick to get him back, nothing worked. I lost him on my PC after a massive explosion made him disappear, but I was able to get him back...

Just wait for a week (in game) and see if he is at Primm or the Lucky 38