Welcome aboard mchargue!
Nice try - you've got part of it together, and if you've just reasoned it out for yourself, that's pretty darn good. However, you didn't hit the home run.
If you've got DishPro LNBFs, you do NOT have any "SW" switches. They'd be "DP" switches. It is VERY important to know which is which before we can figure out what you need to hook up your 921 (except if both current feeds can go to the same place, feel free to hook it up and start playing with it).
You do NOT say what your 3rd receiver is - I saw 508 and 921. I'll assume you're retiring it (or it never existed and you just have a spare feed). However, most likely, any way we configure this, you'll have a 4th satellite feed, so you can keep it.
Oh - and what you say as "3-satellite/3 receiver" is more accurately "3-satellite/3 tuner" as I know you understand - just a minor thing.
SuperDish is not relevant. It's how you get 3 birds ONLY when one of them is 105 or 121 because those 2 are special (FSS not DSS).
You can not use 2 Quads in the same system - and it'd be a waste anyway.
You've GOT to figure out just what's out there, and then we'll pick a switch, and possibly replace LNBFs. I just can't say at this point.
Go
here for some pics that will help you figure out just what LNBFs you've got. We're looking for "DP" or not (just to confirm it) and Single/Dual, Twin, or Quad.
If you DO have DishPro, then no matter what, a DP34 will get-r-done - but a DPP44 might still be the better choice in case E* decides HD will move to 105, thereby requiring you (and me) to get a SuperDish. You do NOT have to use the Separator, but you DO have to use a power injector (no big deal there).