I come from P.R.China. Landed on Thailand on Jun 12 with a tourist visa of 1 month. I have applied for 2 extension already. Both of them are 30 days. Now, my visa will expire on Sep 9, but I want to stay here for another month. Is it possible that I can apply another extension for 30 days? I just read a post and the author said that the immegration officer will just permit a extension of 7 days. Is that true? Thanks!

You have used up the full 90 days and only a 7 day extension would be available and even that would probably require showing an outbound ticket or some other valid reason for stay.