You can tap into any of those pipes.

Each pipe is generally contstructed with a "P TRAP" below the toilets. This "P TRAP" fills with water and the water acts as a barrier to smell.

This is why whenever you smell sewer in a hotel or house, it is from a shower vent or sink vent that has not been used in a while and the "P TRAP" has dried up, thus allowing sewer gas to fill the area.

The only thing you want to make sure of....is the fan is indeed blowing air UP.......not down.

Ideally, buy the same dimeter pipe in the "Y FORMATION."

Cut out the proper height of existing pipe and, like Zandor sad, insert Y FORMATION UPSIDE DOWN, so the air is blow UP the piping.

Make sure it is the proper vent pipe you cut into. Some bathroom fans will exhaust into those as well, and if it is indeed a bathroom exhuast fan pipe, you will want to run that fan full time, otherwise the smell COULD backfire into a bathroom, especially when the outside air is hotter than the exhaust air.

But, most home builders these days build the bathroom fans so they just blow into the attic.