This rumor is probably not true. Valve has not been interested in anything RPG before. Quests sounds ridiculous for Half-Life. Open world sounds unlikely. And inspiration from the Elder Scrolls? That doesn't sound right, Half-Life isn't anything like Elder Scrolls and Valve IS the inspiration, not the one who gets inspired.
And of course it's a full Half-Life 3 game. What, you'll be buying a trial version of the game? And Half-Life 2 Episode 3 would be called that if it really was Half-Life 2. If it's called Half-Life 3 of course it's an entirely full fledged sequel and not Episode 3. Valve doesn't try to trick people.
After 2013? Well, let's just wait and see...
In the event that this is all true, please disregard this entire statement.