The 3" and the 4" appear to be the "main drains" from the upper levels, and if so they CANNOT be used as "wet vents". If that is REALLY 18' from wall to vented stack, then it is too far away from the vent. The downstairs tub is NOT vented, and technically, neither is the toilet.
Wisconsin Plumbing Code Link
In Wet Venting, water closets must connect downstream of all other fixtures within the bathroom group.
added by Terry