Here are some insights into the game (based on a demo): http://www.pcgamer.com/dawn-of-war-3-inside-relics-biggest-rts-yet/
Some of the things to summarize from that:
It's going to have one campaign with all three races, akin to how Warcraft 3 switched between the races.
Base building and larger battles of DoW1 are back.
Heroes or Elite characters similar to DoW2 are also back in a certain capacity.
I care little for the Blood Angels and Blood Ravens give me the gawd damn Space Wolfs or anyone else
I get that space marine had the Ultra Marines which is good the blue color is a nice contrast from the usual red and green of 40K games
Looking forward more to… I am Wrath. I am steel. I am the mercy of angels. DEATHWING! when I was young I played as the Dark Angels and found their chapters history a lot cooler then prissy fucking Vampires like the Blood Angels are:P
Hell even bring back the Salamanders their colors and class were entertaining. I would even buy a Fallen Angel game where you play as a Dark Angel Deserter being hunted by the entire emperor