While there may have been a teaser trailer, it was never actually confirmed that it was Dawn of War 3 until today when Relic released their official announcement trailer. Well, one could argue that the way they made their teaser it had to have been DoW 3 otherwise there would have a riot around the scale of when the Canucks lost in the Stanley Cup finals... but it wasn't official! Moving on!
In the trailer you can see some of the units such as a Wraithknight, Imperial Knight and Gorkanaut. It is also shown that there will only be 3 factions: Space Marines led by Gabriel Angelos, Eldar led by Macha and Orks led by Gorgutz. As for the rocky pit with light coming out of it, it seems to be a major part of the story as all the bodies are gathered in it, which is usually used as power for things such as weapons or daemons in the 40k universe.
There has also been an article by PC Gamer with some info on DoW 3. You can read the full article at their site, but I'll list the main points down below:
Looks to merge DoW 1 and 2
Very colourful
Campaign moves between factions from mission to mission
The plot has something to do with a powerful weapon
Larger battles
Graphics have been done so it's easier to tell things apart
Base building from DoW 1 has returned
Space Marines can load their units into drop pods to transport them
There seems to be a mixture of tanky units to hold the lines with elite units that can break those lines
There are three elite slots that can be filled before the battle. Things like heroes and super units