I have a house that has an old 8-10 inch diameter vent that used to be used as stove/oven vent. I think that newer gas ovens don't require venting like a water heater needs (ie they don't have a "smoke stack"), but this seems to be often confused with an "exhaust fan/vent" that is used to vent smoke/steam/smells from the cook top.
So that 8-10 inch vent rises up out of the roof about 18 inchs and has a cap similar to this:
I think that I can safely terminate a 3inch double walled vent pipe into that riser (I'd prefer not to punch another hole in the roof, since this is a 4 year old "torch on" flat roof that I don't want to screw up).
My plan is to attach a firestop to the underside of the roof so that the vent pipe is snugly held in position and to limit the airflow from the top of the vent back into the attic space. A firestop lets you make a "tight-ish" seal to a hot pipe with out breaking the "1 inch to combustible material" rule for a D-Wall pipe. It looks something like: