I think it takes 5 years and up to finish PhD in theory. Of course it is shorter, if you skip grad classes.
You said part 3 is similar to first 2 year at US, you actually mean that you will take QFT, standard model, string theory which are 2nd year courses at US, right? I don't think you guys do a one year graduate QM, EMM, Stats. mech? This is a standard in every US school, and they wouldn't let you skip, even though you did QFT. In my school, grad student who did part 3 tripos, or EU master, have to take the 1st year standard classes.
Also, they retake QFT in their first year as well. So, I guess they take 2 years US grad classes in one year. But the QFT here may be "different." I remember the guys who did QFT in his own country saying that the one they took are like ~1/2 of our QFT class at US. I read David Tong's QFT notes from part 3, when I took QFT. It basically covers first 4 chapters of peskin. And our QFT covers beyond that, including renormalization, renormalization group, yang-mills theory, functional intergral, etc.
Hope this help