because most likely those three routers have different configuration... i.e. IPs, Channels, name, password, security type WEP/WPA etc.
As for signal strenght, you can use WiFi Range expander or an antenna amplifier or booster.
I am in a building with 3 different Wireless Networks with 3 different wireless routers (Microsoft, Dlink, and Linksys). Unfortuantely, I am on the fringe of reception and need to be able to access these networks. Is there a hardware solution that will allow me to connect with any of these networks on a consistent basis for both internet connection and file sharing?