A year has passed after the events of K2 where the Exile successfully destroyed Malachor and the Sith Lords......
I think K3 will start off with the Exile returning to Coruscant to lay off his charges for crimes against the Republic. He will then reunite all the remaining Jedi on Dantooine to discuss about Revan's situation. Later on, after the decision, the Exile will choose his party members, including ones from K1, K2 and possibly some new ones. He will then dispatch all the other Jedi on key worlds in order to protect them.
After resolving some little problems in Republic space (crime lords, gangs, Sith remnants, etc.) he will head off with his new party into the Unknown Regions. There he will explore various planets in search of info regarding Revan. After 3 or 4 planets, he will meet a mysterious Jedi, who walks in the shadows... he is unknown to the rest and reveals he is the apprentice of the Dark Lord of the True Sith...
Combat... bla bla bla... and the party ends up on some world (like Ziost for example, the homeworld of the True Sith)... he ends up finding some starship (who he finds out is Revan's) and blows it up and he arrives on the surface and finds Revan working to build a super armada of Sith to launch on the Republic.
A huge war engages (like the one in ep2) and all the available Jedi come to help. Eventually, the Exile arrives in some temple or tomb and finds a source of great DS power... but Revan is also there... here the Exile has to choose what path he wants to follow.
The two speak about their past and mock each other and finally they fight...
the Exile wins... but depending on your choice: LS or DS... if he is LS he asks Revan to save the galaxy by destroying the immense source of DS power on the planet (he also tells him that he won't be forgotten)... Revan destroys the source and dies in the explosion as the entire temple blows up and the Exile and his friends leave the planet's orbit. They return home on Coruscant to celebrate the defeat of the True Sith but they also commemorate the death of Revan, the true destroyer of the Sith and saviour of the galaxy... Nice ha?
But if the Exile goes on the path of darkness... he will kill Revan and use the source to empower the Jedi and his Sith followers and rule the galaxy...