Saw this at a friends house. Trying to learn and understand. This tub drain goes into a 3" stack that goes thru the roof. Above the tub sani-tee is a toilet, then the sink before going thru the roof. All fixtures are within 30" of the stack. Is this considered a s-trap? Does the tub or sink need separate vents? Is it OK for the toilet to drain past the tub or does that create problems? Thanks for the help.