According to KoreaGames.com, Lineage 3 is being worked on by NCsoft. From their newsletter:

'Lineage 3', expected to spearhead the boom of MMORPGs on the heels of 'Lineage' and 'Lineage 2', is now under development at the NCsoft Studio in Korea. No more details have been publicized yet except for the fact that the game will be an extension of the same episode shared by other 'Lineage' series games. 'Lineage 3' will be unveiled with much pomp and circumstance in the first half of 2007.