How to stop 2nd floor toilet from bubbling when 3rd floor toilet is flushed?
To be it sounds like the back flow preventer on multi story building is partially blocked open. Or a venting issue. The easiest is to check the main vent stack and make sure it is clear. I have found main vent stacks that had construction cloth covers blocked with spider Webb’s enough to cause problems, in another case a dead squirrel, and several others bird nests and dead birds in the vent pipe. I would check the cap and make sure the vent is clear first, I could not believe spider webs could cause a problem but did on a 3 story above 2 below grade building.