Ok... in first place, the Half Life 2: Episode Three will be released in the 2010, aproximately, as Valve said in one of it's pages, other thing: THERE IS NOT such thing as an Episode 3 Oficial Trailer, yet, down here there is a "episode 3 video, which is absolutely a Gmod video, a game which i will talk about later (i'm sorry that i cant upload any video here, just the links :p )

http://www.youtube.com/watch?v=ORhGbjUxHIQ

This is, obviously, a Gmod or a SDK video, i will show you the reasons why it is what it is:

1) Alyx, at the beginning of the video, and when the helicopter crashes, just says "Gordon!", and "Hang On", of course this sounds are for Ep1.

2) When Dog is going to move the helicopter, he just uses his gravity hand to pick an object that is constrainted with a rope O,O (note for the editor: THERE IS SOMETHING CALLED "WELD WITHOUT ROPE, JESUS"

3) At the beginning of HL2, Ep1 and Ep2, the Gman always appears, here, it doesnt.

4) When the player is going to enter in the heli, Alyx gets in first, anybody that has played ep2 will note that those are the same movements that she makes when is going to enter the Jalopy (or Muscle Car), and she seats out of the helicopter, then it fixes and appears a little more up.

5) This is the most stupid fact: Valve will never waste their  Source Engine, which was used on the Lost Coast part of Half Life 2, to only disappear the screen when the helicopter takes fly, to reappear "two days later" in single map, with only 3 houses, in which map u can see the edges of it, and without any effects of objects  in the snow, COME ON! the map looks like a box, have u seen the precipice?, it's much more straight than a cube.

6) And a lot more of things that i won't type here because i haven't seen all the video, heh, just a few parts, but im sure that if you see it, you will know i'm right...

 Now, here is an Oficial Concept Art from Episode Three. The piece--created by Valve artists Ted Backman, Jeremy Bennett, and Tristan Redford--appears to show Half-Life hero Gordon Freeman within a large Citadel-like environment, a Combine Advisor looming ominously overhead.


Full Size Image: http://www.shacknews.com/screenshots.x?gallery=10212&game_id=5486#img122102

This exclusive render showed what appeared to be a group of Advisors floating in what i presume to be the icebound Borealis, a research vessel that was once operated by our friends at Aperture Science.

 
 Full Size Image:  http://www.gamesradar.com/images/mb/GamesRadar/us/Games/H/Half-Life%202%20Episode%203/Everything%20Else/Concept%20Art%200710/2008-07-10/~$GUS.Feature.HL2E3%20Concept%20Art.2--screenshot.jpg

  Now, here you can download the song: Path To Borealis, by Viktor Antonov, in MP3 Format:

http://sharovar.com/files/NDQzMS4yNzIzMTQ1NC4yMTM0ZDdlNGYxMGQ/Viktor_Antonov-Path_of_Borealis.mp3

 This is a "picture" from Half Life 2: Episode Three, this one is obviously a montage:

 
 Full Image Size: http://img242.imageshack.us/i/halflife3pc0.jpg/#q=Half Life 3 (copy and paste this one)

 If you pay attention, you will note that the headcrabs and the explosive barrel are a montage, and from Half Life 1, i know it's obviously, but i have seen a lot of people that thinks that it's a real Ep3 image.

 Thats all for now, stay tuned for more info.