When ever either shower is used in upstairs bathrooms (they are side by side rooms), the bedroom next to one of the baths smells of sewer gas (septic tank).Also that area over the room in the attic smells of the gas.Below these bathrooms is the laundry room and the same results happens when the washing machine is used. Kitchen sink and dishwasher use OK. Roof has a 6" vent and a 3" vent . The 3" vent runs through the wall of the stinking room. Smell goes away over night. Downstairs bath has no effect on smell and works fine.

Tell me more details regarding the evnt through the wall, where does it go after that? You clearly do have a vent issue, it could be partially clogged causing the traps to siphon dry or you may have a pipe that opened at a seam. Try rodding from the roof to the basement floor plus 15 ' or have a smkoe test done by a plumber. You could try to do a smoke test yourself.