Plumbing Vent size recommendations
Check on the exact wording there ........
Our code (OR) is worded that the aggregate cross sectional area of vent penetrations must be at LEAST the same cross sectional area as the MINIMUM REQUIRED building drain.
So, here, you could have two 2" vents and one 1-1/2" vent to meet this requirement for a 3" building drain.
This requirement is for the entire house, mind you, so if you're ADDING a bathroom that is tied into the building drain of the original house, you can count the other roof penetrations as well, in which case, you may only need to stub out a single 2" vent, which would save you a considerable amount of money not using any 3" fittings....