Routers are normally used to connect computers on different networks/subnets; if you really want all of the computers to be on one network, replacing routers B and C with switches would be the recommended way to go.

However, if you connect the A router to one of the LAN ports (not the WAN/Internet ports) on the B and C routers, you'll only be using the switch portion of the B and C routers, bypassing the hassle of disabling NAT and the other configuration tweaks that it would take to get all of the routers on the same network.

In that case how can we configure it. I mean can u explain how can we configure that.

Ok:

Broadband routers like the ones made by Linksys and Netgear are really two devices in one: a router, and a switch. The LAN ports (the ones you connect your computers to) are part of the switch section, which doesn't really need any configuration; it just ties together all of the computers connected to it. The router portion of the device manages traffic between the external network (connected via the "WAN" port) and the computers connected to the switch portion.

The basic connection scheme in your case would be:

Internet
|
|
V
[WAN Port ] Router
[LAN Ports] A
| |
| |
| |
| | Router B
| L>[LAN Ports]-----> Computers
|
|
| Router C
L>[LAN Ports]------> Computers

*All connections to/from Routers B & C are on LAN ports.

If Router A is configured to assign IPs to computers on the network through DHCP, there shouldn't be any more to it than that.

If you are assigning IP addresses manually, all computers IP addresses should be in the same network range, and the LAN-side IP address of router A should be entered as the gateway IP on each of the computers on the network. Obviously, with manual configuration you'll also have to enter the correct subnet mask and DNS server IP on each of the computers as well.