Getting address of gateway and local network

I have 3 PCs connected to each other as a LAN. Call it PC A, B and C.
PC A has a modem for internet connection and PC B & C use PC A as gateway to the
internet without using any proxies.

From the PC B or C, how do I get my LAN's gateway address (PC A) and the gateway
address used by PC A ?

Thanks in advance for all replies.