Nick finally announced when they're releasing the last episodes of Avatar.
Book 3, Episode 16 (The Southern Raiders) - July 17, 2008
Book 3, Episode 17 (The Ember Island Players) - July 18, 2008
Book 3, Episode 18 (Sozin's Comet, Part 1) - July 19, 2008
Book 3, Episode 19 Sozin's Comet, Part 2) - July 19, 2008
Book 3, Episode 20 (Sozin's Comet, Part 3) - July 19, 2008
Book 3, Episode 21 (Sozin's Comet, Part 4) - July 19, 2008
Watch the Book 3 Finale trailer here (new footage).
Notice book 3 has 21 episodes instead of the usual 20 like in the first two books. The last four episodes (18, 19, 20 and 21) and going to be combined to make a two hour special.
Also, it might interest you to know that book 3 is the last season of Avatar. So it'll all be over by the 19th of July, but the creators have said that 'other stories will be told in the Avatar world' so basically they're saying that there is going to be a spin-off series or episodes.
There is also going to be three 60 minute long animated (like the series) Avatar films as well as three live action Avatar movies. The live action movies are going to be directed by M. Night Shyamalan (The Sixth Sense, Signs, The Happening). Each movie is going to represent the elements like the series did (first movie - water, second movie - earth, third movie - fire). The first movie is currently in casting and set design and is due for release on July 2, 2010.
NOTE: The air dates above are American air dates, but there is quite a high chance that is when they'll air in the UK as well. If not, they'll be on the internet then anyway.
http://www.avatarchapters.org/