Sega of America has recently registered a domain for "dawnofwar3.com", something that certainly suggests that there's plans to release a new chapter in the Warhammer 40,000 series made by Relic Entertainment (originally back when they were a part of THQ).
No details have been offered so far, and Sega of America haven't officially announced anything regarding Dawn of War 3, but the domain registration seems to have been made by Sega's George Bueno, which means they're either thinking ahead and protecting the domain name, or they're preparing to unveil the game in the coming months.
Relic Entertainment has been working on a unknown project since releasing Company of Heroes 2 and its expansions, so it's certainly possible that they're working on the next entry in the Warhammer 40K real-time strategy series, but until we hear more on the matter, perhaps it's best to be conservative with our expectations.