Ideas how to convert this flange to a single 3" Pipe

Several ways. buy a big roller from harbor Fr or Nothern. then buy a shrinker. Then spend time learning the machines. Buddy has them, he made all kinds of sheetmetal for his 69 Charger.

Or just get some 12 or 14 gauge, cut it up, bend it in the vise,hammer it over a 4" metal pipe, and start welding.