This is what I see.

1 - It might be a 2" connection for a sink drain or shower drain, or a drain and a vent. A toilet would require a 3" drain fitting and a 1.5" or 2" vent.

2 - Probably a 2" floor drain, with an air admittance valve for a vent.

3 - 3" drain pipe from bath above. Unusual because it does not have a cleanout in the pipe. How would you snake out a clog? It may be just a vent pipe, but that would not require a 3" pipe.

4 - 3" drain from two toilets above. May be the main plumbing stack.

5 - 3" drain pipe that appears to also serve as a water softener drain (can't really see for sure). Sump pump next to it.

I would pull off the caps shown in picture 1 and try to figure out where they go. Then pour some water down them and see whether they drain.

Look for your main water shutoff valve inside the house and try to determine if the main sewer pipe also comes in under that wall. Installing both (water in and sewer out) along the same path is fairly common. Then look at the other three 3" stacks and try to figure out how they might connect to the main pipe. That may give you an idea of what is under the slab and where you could connect new drains.

Bruce

Thanks for your feedback, Bruce.

The two caps in the first image both indicate being 3". As you suggested, both could possibly be drain pipes. I will check the flow and see if I can determine how the pipes are connected to the main sewer line and if they are drain or vent pipes.

Thanks again!