Plumbing – Can a toilet vent be upstream

905.2 Grade.

All vent and branch vent pipes shall be so graded and connected as to drain back to the drainage pipe by gravity.

(2006 International Plumbing Code).

This might prevent you from doing what you want, since your lower horizontal connection would have to slope toward the stack. Not sure if this is allowed or not, but it might be something to look into.

So for those out there looking for an answer here is what my plumber did:

The toilet, sink, and shower were all fairly close to each other (in my case about 6 ft). He installed one AAV inside the cabinet under the sink. That is the only AAV installed. He said it has now become a "wet vent" to the toilet and shower drain.