A lot of people are tossing around the idea that there will be a third installment in the Portal franchise. I'm here to use logic to shoot that claim down. I'm also going to use logic to explain why half life 3 isn't announced yet, so prepare for my theories on what Valve is doing (besides making hats).
Bear in mind, this will contain spoilers about both Portal and Half Life (albeit less about Half Life, because admittedly, I haven't beaten HL2 yet.) You have been warned.
Half Life, a beloved franchise by pretty much everyone. Even though the games are dated, they've aged quite well, and are definitely worth the $15 price tag for both of them on Steam. Half Life 2 also spawned the Source engine, which Valve has made all its games in since 2004 (excluding maybe CS:GO, but I'm not sure). Nowadays the Source engine is old news. Portal 2 looks amazing for something made in it, but the simple fact is that the Source engine is from 2004. The Gamecube was current gen in 2004. Now imagine someone making a big budget game that people have wanted for years on the Gamecube. Sound absurd? Yes. Even though it's been updated, it doesn't really work that well on modern hardware. Nothing against it, but it's showing its age. Valve probably isn't making Half Life 3 until they have a new engine and some practice using it. They're probably only waiting until they have all the technology needed to make something amazing. With all the first person shooters around these days, they're going to need to make something revolutionary to live up to all the hype. And I'm sure they will, because Valve has never failed on that front before. Basically, what I'm saying is that it takes time to put together something wonderful.
Now onto more guessing based on the lore of the two franchises.
Portal and Half Life take place in the same universe, and in the same basic timeframe. It's common knowledge which can be gathered by bits of dialog in the two Portal games. In the end of Portal 2, the story is completely wrapped up. Wheatley is in space, GLaDOS has robots that she can test, and Chell is... free? Hardly. Chell has just been locked out of a safehaven from the horrible things going on in the world. She's unarmed and away from civilization in a world with an oppressive government that controls an army of Combine soldiers. Unarmed and locked out of Aperture, Chell would most likely die before reaching civilization. By the off chance that she DOES reach civilization, it's unlikely that she would stand for the oppression of the society that she would find, and would therefore join the rebellion. But then the next Half Life game would have to be a crossover, which I doubt they'd do. Because the Portal games are suitable for a younger audience and I think parents would have a hard time explaining to their children that they're only allowed to play the first two games in a franchise.
Basically, Portal is probably going to just be a game with a sequel, not a trilogy or anything more.
If by some strange coincidence the next big Valve game is a Portal-Half Life crossover, then heck, I'll preorder it even if it costs $60. But I think that's pretty unlikely. I don't think it's IMPOSSIBLE, and it would definitely sell extremely well. But the most logical thing to assume is that Valve will release a new game in a new engine, then release Half Life 3 roughly a year later. And maybe another co op Portal game without the emphasis on story, if we're lucky. I'm excited to see what Valve has in store for us, and I know that whatever it is will be great.
And also, half this crap might be disproven by the ending of Half Life 2, but this is just my theory so far.