Three and a half bathrooms, that's eleven fixtures right? You have to come through the roof with 3" (here you do at least). So you would have a pretty unsightly house if all eleven of those fixtures have their own 3" vent coming up from the roof. Completely legal but wrong, and ugly. You didn't replumb the whole house using AAVs did you? That's probably not going to fly with the inspector.
The point of the stack is it's a place for the vents and/or wastes from the fixtures to dump into then come out of your roof with one 3 or 4in line instead of eleven.