how many fixtures on a 3" stack?

Hi, Jason here and a remodeling/tile contractor by trade.. Am doing a bath remodel which looks like the drawing I am attaching, but have a couple plumbing questions. It seems someone has updated this house by retrofitting 3"PVC drain stack to replace the original Cast (there is still a 2 foot section.)
This drain line runs down the basement walls probably 30 feet before going out. What I am wanting to do here is seperate this bathroom into two with a wall right behind the tub. I will be adding another toilet, and a shower to this setup. Can the 3" stack support all of this ok? Just wondering because I usually see 4" in most houses. Also there may be another vent in the wall where the double sink is but I am not positive. If not can I use one way valve vents under the sinks in both baths to offset the lack of venting?

Thanks

There is NO question about WHETHER you can do it. BUT, HOW you do it will determine the success or failure of your project. The drawing you submit to the building department, along with their inspections, will ensure that you do it correctly.

No building department or inspections here, out in the middle of nowhere. I am simply worried about function. Am I ok to add another toilet and shower to that drawing with a 3" line? Also where would I need another vent...?

You CAN add another toilet and shower, but we have absolutely NO IDEA how, or where, you intend to put it and thus no recommendation about where your vents would be, but you absolutely DO need them. THeroretically, you could change the dynamics enough that even the existing section would need revision to its venting.