Fellas,
You are right about the angular spacing not necessarily equating as one departs from a standard 20" dish. As the dish surface is curved, the signal (from one direction) is reflected to a focal point. If the dish is a perfect parobolic dish, all the refected signal arrives at the "Perfect" focal point, when the dish is aimed at a single source. As the source deviates from the designated aim, some (less than the ideal) of the signal is concentrated away from the focal point to a secondary focal point. The dish 500, dish 1000 and DirecTV 3 sat dishes all utilize less than perfect geometry to balance the signal reflection amongsth the 2 or 3 satellite signals.
Short of measuring the dish, calculating its curvature and mathematically calculating the optimal location for spacing dishes, we experiment. I picked up 129 (with a 3rd lnb zip tied to my dish 500) and had a higher signal strength than most of us (including Greg M) in the pacific northwest do with a dish 1000. Since Greg M is experimenting, I added some additional ideas to try. Adjustability is a good thing as long as one has the time to adjust.
If I'm not mistaken, the answer to connecting 3 DP duals (or singles) would be a DP34 or a DPP44. If using a dual, only 1 coax is needed to the switch.
Mark.