I'm looking to cut a 3" hole into a cast iron waste stack so that I
can attach a 3" PVC adapter to the waste stack in the basement.
I will then attach a separate 3" PVC riser (from the 2nd floor) to the
PVC adaper in the basement.

I'm wondering if anyone here has any similar cutting experience that
they'd be so kind as to impart.

The best advice I have so far is to drill a series of holes on the
circle that rpresents the new opening in the cast iron and then
connect the holes (scroll saw, sawzall, rotary cutting tool?).

Thanks for your consideration!

Al