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

is that ehaust? if so, take a piece of 3" pipe, and squish the one end into an oval. You may need to cut some slits in it to open it up to get it to fit on your flange. Once it is on there, you can weld the slits back up. If the pipe is too big, cut your slits so they are wider at the opening and work it smaller a bit. That is what I would do.. may be a bit crude but should work fine.