I needed 2-45 degree bends for the DP (word from the wise 3" is too big for most cars) I was able to free the flange off the old dp, remove all the welds, and it fits perfect on a 3" pipe. I did not use a flex, and it's been crack free for over a year.
The converter to resonator can all be the same size str pipe. To lower reversion, I went from a 3" cat into a smooth reducer to 2 1/2" for a resonator, then back to 3" for the bends. After the resonator, you'll need 2-90 degree bends, a short st pipe, and a 45. You need to make a split, but it's easier to buy the Flowmaster one. Then you need 4-90 degree bends to do the two mufflers. I would say definately go with a resonator AND mufflers if you go to a 3" pipe. It's going to drone and be raspy w/o it.