That's what they call CSR roulette. The Dish 1000 upgrade might not be free to you since you're an existing customer (YMMV), but it does exist and since September 27th it's the default method for must carry locals if you're in the right DMA (and you are).
Either a DP44 or a Dish Pro Plus Twin LNB will be used on the dish, depending on how your stuff is installed. Chances are if you already have a DP34, they'll just use that, but if you don't it all depends.
A DPP Twin LNB will drive 2 receivers since it has 2 outputs. With the proper splitter, yes they'll drive a 625, 522, or 942 with 1 cable running from the receiver to the dish. However, it only works with dual tuner receivers.
If you want to do HDTV, you might want to get a 942 for your first 2 TVs since the first will be HD and the 2nd output will be regular TV. That's a $250 upgrade to lease ($5 a month and $250 one time), but well worth it anyway. You could also just get a 522/625 and an 811 (non-DVR) for your HD viewing.
Either way, a DPP Twin LNB will only allow 2 receivers, a DP34 will allow 4 but dual tuner receivers take up 2 outputs on it, a DP44 will allow 8 if they're all dual tuner (right?) -- but it has 4 outputs, so you either can use 1 for each single tuner or each dual tuner receiver.
With a DPP twin LNB, you won't have a switch. With a DP Twin LNB, you will either have a DP34 (3 satellite locations), or a DP44 (4 locations). Unless you have a lot of receivers, there's no need for the DP44 since you're only going for 3 locations anyway and the LNB can handle that.