When does the 3rd Trimester Start?!?

Cheyenne • I am 23 and I have a two year old little boy and a 3 year old little girl and pregnant with #3!
Does anyone know when the third trimester starts? I have read conflicting info over it, some say 25 some say 28. I am 25 weeks today and I just wanna know if I am in the 3rd trimester yet lol I know it's silly but it's still cool to say oh I'm in my third trimester :) anyways has anyone had their doctor tell them when it officially starts? I don't wanna bother my doc with a silly question like this.