In California, You can plumb a three bath home with one 3" vent.
Most of the time, we would use
2-2" vents, two or three bathroom sets and the laundry thrown in.
1-1-1/2" vent Kitchen
I have only used 3" vents on the West Coast on Condo and Commercial projects.
On homes, I use the smaller vents.
Most of the time, the kitchen is isolated so there isn't much else that can be added.
A 2" line will vent three bathrooms, as long as there are other vents in the system to equal the "area" of one three inch vent.