I'm turning a 5x9 porch (over shallow crawl space) into a three-quarter bath with lavatory on east wall, shower across south end, toilet on west wall. I'd like to bring the lavatory drain and shower drain together into a 2" wye, then that wye into the two-inch wye neck of a 3x3x2 fitting that will aim toward my 4" main drainline in the basement. The lav/shower drain would enter the 3" main line a foot or so downstream of the closet bend. All the drains will be horizontal (lying under the subfloor) with proper 1/4" drop per foot until the 3" drops vertically into the 4" basement plumbing. My question is regarding venting. With such short drain lines (nothing more than five feet apart) can I get by with one 2" vent in a wall (either for the lavatory or toilet) serving all three fixtures, or do I need separate vents for each? The vent could go straight up through the roof on either the lav wall or the toilet wall. In other words, do I need three pipes up a wall or will the one 2" vent pipe that joins the drain under the floor be sufficient to vent all three fixtures? I know it is OK to run separate vents for each fixture upward to join one large through-the-roof vent above the ceiling; what I'm asking is whether I can avoid two of those three pipes going up a wall. Is this at all clear what I'm explaining? FWIW, there are no codes or inspections or permits required; I'm in an area where no one officially cares, but I try to do work that would meet typical codes. Thanks for any advice.