OATEY 43789 3 in. x 2 in. PVC saddle tee kit

That's exactly what the Oatey 43789 saddle tee kit is for. It solvent welds onto an existing 3 inch or 4 inch Schedule 40 DWV line and is clamped down with the metal V-bolts, washers and nuts that come in the kit, giving you a 2 inch outlet. You cut the tap hole in the line rather than cutting out a section and rebuilding it with a wye and couplings.

Drain, waste and vent only. The 43789 saddle tee withstands working pressure up to 10 PSI, which is a gravity drainage figure, not a supply-line one. Don't put it on domestic water, compressed air, or any pressurized branch.

Both. The kit is sized 3 x 2 inch, but it's made for tapping into either a 3 inch or 4 inch Schedule 40 DWV line. The outlet stays 2 inch either way.

Yes, with a conversion gasket. The 43789 saddle tee is rated for use with a conversion gasket on cast iron; without one, it's a solvent weld connection and solvent cement doesn't bond to cast iron. Order the gasket separately, it isn't part of this kit.

The kit gives you the PVC saddle tee itself plus the metal V-bolts, washers and nuts that clamp it to the line. Pipe is not included, and neither is solvent cement or primer, so bring those to the job. If you're tying into cast iron you'll also need a conversion gasket.

The tee takes the incoming 2 inch line inside its socket, so a plain pipe end drops straight in. It will also join to a Schedule 40 DWV fitting if you're coming off an elbow or a trap arm rather than a straight run of pipe.

Judgment call that belongs to your inspector, not the fitting. Mechanically, the 43789 is built for exactly this: tapping a 2 inch branch into an existing 3 or 4 inch Schedule 40 DWV line, solvent welded and V-bolt clamped, at up to 10 PSI working pressure. Many jurisdictions restrict saddle fittings on sanitary drainage and want a cut-in wye or sanitary tee instead, so check your local amendment before you drill the line. On repair and remodel work where the line is buried in a slab or a finished ceiling, the saddle is what keeps you from opening the whole run.

It's a one-hand part. The saddle tee kit weighs 0.85 pounds complete with its V-bolts and hardware, so the install is about hole location and cement work, not handling.