Thanks for the replies. Reach4 - I'm embarrassed I didn't think of that! The fitting is magnetic, so, C.I., I guess. So is the flange (unpainted part) at the top. It looks like there's a copper fitting that fits into the flange, and obviously, the pipe is sweated to the copper fitting. I see a small stripe of what looks like black paint on the copper fitting where it goes into the C.I., like maybe it's coated with something to prevent it from corroding where it contacts the C.I.? The connection between the copper and the C.I. is really clean - no pipe dope or threads or anything like that is visible.
The three inch is a short (~5') toilet waste line that currently has three 90s and a 45 before it gets to the fitting in the picture. It's really prone to clogging, and I'm pretty sure the bottom of the pipe is getting thin. I've had copper pipe split in other areas of the house. When we moved in, I found a lot of caustic drain cleaner left by the previous owners. I have a feeling that they used a lot of that, and I understand that's not so good for copper, which might explain why I'm having problems now.
If I remove the copper, I can make it a much more direct run for the toilet. Also, I want to add a combo to the vertical 3" to pick up a laundry sink & washer from the floor above. Right now, the laundry comes through the sidewall under the sink, and it drops into the the horizontal 3" with a T. Not quite ideal... I want the laundry drain to come through the wall behind the sink so I can put in a base cabinet and sink/countertop.
