1. Valve isn't going to release the most anticipated game in history on a new engine that doesn't have at least one game on it first to see how things work out with said engine. The first game on the source engine wasn't HL2, it was CSS. So don't get mad if Valve's next game is L4D3.
2. ".RU"? Seriously? Why would Valve give their website a Russian tag? That makes no sense whatsoever. Obviously a Russian, Ukrainian, etc. prankster playing a joke on fans.
3. Valve doesn't work on one project at a time. They work on multiple projects at once. Employees simply pick what they want to do, and do it. So Valve may be working on L4D3, HL3, and Portal 3 at the same time. Nobody knows.
4. Yes, the picture of the L4D3 document was pretty convincing, but we can't confirm anything yet. It would certainly suggest L4D3, but we can't be sure of anything. In my opinion, I think they're working on it. But so what? It's not like anyone can confirm a release date or anything.