Does Luke leave Professor Layton in the third game?
Luke does leave at the end of the third game, you can see a
video of him leaving on youtube. However it is all in Japanese, so
I'm unable to say why he does leave. Professor Layton 3 and the
Last Time travel has not been released outside of Japan at this
time. However despite the games having slight differences in
different regions (the odd puzzle here and there, some characters
names etc). The main plots have all remained the same. So if you
are able to find out what happens in the Japanese game, the chances
are they won't change such a huge plot development when localising
it.
This doesn't mean Luke won't be in any future Layton games, the
fourth game to be released is actually set before the Curious
Village, and sets up how Layton and Luke met.
It marks the start of the another set of three games, all set
before the Curious Village according to a March 11 2009 news
release by Level 5 (the creators of Professor Layton).