First, I am assuming that the transmitter you described transmits on the 802.11g standard. That is essential for using low-cost routers. You will need high-gain signal antennae's on ( B ), with a signal repeater/booster access point there also. The idea is to set up ( B ) as an access point to get the weak signal, boost it & re-distribute it. Routers at the other homes would do well with high-gain ears as well. The stated spec. for signal range on a standard signal is 300 yards, but as you mentioned terrain and other factors affect the signal as well. The straight-shot signal from (A) to ( B ) may work. Adding highest-gain rabbit ears available may make it all work. It's the distances involved that are working against you most.