How to stop downstairs toilet from bubbling when upstairs toilet is flushed?
It seems to me that what is happening is when the 3rd floor toilet flushes, the volume of air in the 3 story length drain pipe needs to go somewhere, and it is venting out through the ground floor toilet.


I believe your observation is correct. The problem is, you have an AAV (Studor valve) on the toilet to vent it. This allows air in but does not allow air out. Otherwise you would get a foul smell from sewer gas.

So, when the upstairs toilet flushes a large volume of water down the vent stack it forces air up through the toilet since it cannot go up through the AAV.

The only really good solution is to add a vent to the downstairs plumbing that goes all the way to the attic and joins the house vent stack there.

So, you can live with the bubbling or install a standard vent for the toilet.

Good luck!