Attaching photo for background. Will try and add additional photos.

House is two story with finished basement ...lot slopes away so septic tank and field all below grade of the basement bathroom - no septic pump. House has two main vent perforations of the roof. Photo shows the one I am working with in attic ... 3" and has been terminated at ceiling level -will have to be cut near roof or completely removed and replaced.

Some more background - the new toilet and shower flow to the same horizontal waste pipe in a shared pipe. The tub and vanity sinks via another line to the same horizontal waste pipe higher than the toilet/shower.

Flooring, shower base, etc all in now, so can't change anything without ugly consequences.

The main bathroom stack connected into the attic stack shown in the photo (serviced bathroom - connected down to horizontal waste pipe and up to vent through roof) - it was removed and temporarily terminated at ceiling level in prep for complete removal. There are a total of three vents in need of connection to the through the roof vent ... 1. A single 2" from somewhere from first floor, 2. A 2" three-into-one from somewhere downstairs/adjacent bathroom, and 3. The new 1 1/2" vent that services the toilet and shower.

Since I can't tell where vents 1. and 2. go, is it reasonable to use vent 3. as the vertical "stack," transition it to 2" in the attic and then connect 1. and 2. to it in the attic, removing the existing 3" and staying with 2" through the roof in a new cap? Or, simply transition the 2" to "3" and connect it to the existing cap.