I think you could refer to this website's introductions about these three cities, including transport, food, shopping, nightlife and others. Very detailed.

I think 18 days are more than enough, you may also go to Shangri-la and check the Meili Snow Mountain, the view is really stunning. There are regular flights between Kunming and Shangrila, so I suggest you can go like Thailand - Kunming - Shangrila - Lijiang - Dali - Kunming - Thailand. Have a nice trip!