"will have three sites(pc) to start with, about 250 yards/meters apart" is far beyond wifi specs.

But there is hope. In about post 17 of our forum sticky I posted how to use normal wifi routers as WAPs. You could use your EVDO ROUTER and use WAPS wired to your EVDO router. I decline to explain what a WAP is or why this fixes your system. You must be conversant with this technology to the point you get it with the words so far or I would be entering some tarpit.
Bob