Uour old farmhouse has one bathroom on the main floor. The 3" drain becomes a 2" vent stack at main floor level. The main floor bathtub abd sinks share that drain and are connected downstream of the WC drain. We want to add a half bath to the upstairs and the 2" drain is already up there and connected to the main floor sink drain, though it doesn't look like there's a vent going up from there.
Do I need a vent upwards from that upstairs 2" drain?
Also, Im going to extend the 3" drain up to the third floor and tie in the upstairs back-outlet WC and continue the vent at 2" upwards from there. Do I need to enlarge that to a 3" vent if both toilets are going to share that drain? I also plan to add a third toilet in the basement that would share that same 3" drain. Would that create a problem? The basement has two drains leaving the basement through the sidewall foundation just a few inches above the floor, the 3" we're discussing and also a separate 2" that I plan to use for a basement kitchenette a lavatory sink and a shower and possibly the washing machine. Do you foresee any problems with this plan? I'm sure I'll need to run a separate vent for the basement 2" system, but will that be too much stuff on the 2" line? PS: This old house has a separate gray water system at the kitchen end, and I can move the washer to that system if necessary and possibly even the shower if I use a basement pump. I'd rather NOT pump mulched blackwater in that system, as I don't think it's designed for it.
Thanks, DL

Yes the upstairs two inch drain should have a vent. You will be just fine using two inch for two toilets. And in my area you can have up to three toilets on a three inch horizontal waste line, or four toilets on a three inch vertical waste line, so you should be just fine with three toilets and all other fixtures on the same three inch waste line, as long a they are all vented and tied together properly, you should be all set. Good luck on your project and come on back for any further assistance. Please let us know how things work out. Lee.