The app for this website says the third trimester starts at 27weeks :) I have 2 other baby apps downloaded and they both say 27weeks to :))
My Dr goes by 27, so that's what I go by
From what I've read, I'm going by 27 weeks as the start of the third trimester.
Check with your doctor. I go by what mine says
28 weeks is what both of my doctors go by.