I just went up to the attic to check how everything is vented, and this 3" pipe is the main vent stack. But from this pipe, there is a 2" vent offshoot going into the second bath. The kitchen and downstairs half bath are below the second bath. So maybe the 2" vent is venting both second bath as well as downstairs. The reason i think that is true is because all three old fixtures in master bath (the one I'm renovating) were plumbed to drain into this 3" pipe, as seen below. First connection is for toilet, second for sink (sink has separate vent), and third (bottom of pic) is for old tub. Builder wouldn't have drained toilet into vent stack for downstairs fixtures.