Personally, I always thought that they should name them like they did the seasons. So have The Last Airbender as the series name and name the movie
Book 1: Water
Book 2: Earth
Book 3: Fire
Maybe leave out the books part of the title, because it might confuse people. However, they are obviously not going to do this. This method works for the same reasons it worked in the series. The reason Mike and Brian named the season that. They are the elements Aang needs to learn. It just work.
The thing I don't understand is, are the other 2 movie not going to have The Last Airbender as part of the title. I think it would be a really horrible idea if they didn't include it. Otherwise no one would know the movies are related. And if they don't include that part of the title it makes naming the other two movies even more difficult. It worked for The Dark Knight, but that's a unique case. Anyway, assuming they keep the series name in there, my other choices would be
Second Film: Crossroads of Destiny. (This is just awesome as a subtitle.)
Third Film: Sozine's Comet
I would have said Day of Black Sun Because it's the coolest title of Season 3, but it's not as important as the comet and it ultimately fails, so naming the movie after it wouldn't work well. But it does sound the best.
Personally I think they should give the first movie a subtitle too. It balances everything out. I think that if you know you are making a trilogy then naming the first movie is a must. All the greats have it, Star Wars, Lord of the Rings. If they ever decide to give it a subtitle i would call it, Siege of the North. But that probably won't happen. Oh well. They will probably come up with their own titles as to distinguish themselves from the series. But i think it would be better to take the show titles. One, they are just good titles and two, it would be a nice nod to us fans. I would still prefer the Book names though. Giving them other names just makes it to serial. Which works for Star Wars, but the book names would set The Last Airbender apart. It wold make it look like it's all thought out and a little more stylized I guess. Hope that makes sense.