Your 3rd receiver is going to need a DP34 switch if not a DP quad, and (best practice) a DP twin rather than the DPP twin. (Some say the DPP twin will work fine in this scenario, but you will have to unhook that wing dish coax and run it to your switch rather than the DPP twin. An alternative would be two DP duals to go on the Y adapter on your Dish 500. BTW - you didn't mention whether you have a Dish 500 or not, nor at what your 2nd dish is pointed.
Anyhow, you need one coax per satellite location cabled up to a DP34, and then you can hook up any combination of 4 DishPro tuners on the outputs of the DP34. A single DP34 can only service 4 tuners, so that's no more than a DP quad. But the DP34 can be cascaded via passthrough to as many as 2 more DP34s, or a DPP33, or a DPP44. You can really go crazy with multiple receives by cascading these switches.
BTW - I do not think a DPP quad exists...