Actually, each of the three methods gets used. Generally, #2 is what works best for me. However instead of nailing a 2x6 to the existing roof, consider that the entire cut section of the porch rafter needs supported. This may require using a 2x12 or more depending upon the slope of the roof. It is also good practice to remove any shingles that are directly under anything nailed to the roof. This makes reroofing much easier and allows the nails to get a full grip.
just my .o2, Doug