I am using 03 LAN ports in our system using logical names as eth0, eth1 eth2. All are working fine. But when I change any of the LAN card, system detects it as the next logical name ie eth3 because the change of hardware address of LAN card and by default no IP address in assigned to it.But if I chgange the hardware address in corresponding file in /etc/sysconfig/network-scripts/ifcfg-ethx mannually then it takes its old logical name ie eth0/eth1/eth2 and work as normal.

The requirement is that whenever any LAN card is replaced with new one it should retain it logical name as eth0/eth1/eth2. These names also should be fixed with PCIe slot wise. We can say slot 1 card is eth0, slot 2 card is eth1 and slot 3 card is eth2.

I think the system detects the LAN card with its serial number or type of network chip.
Check the kernel source code for that or try to contact the developers of the code for further details.

udev is the normal answer, but depends on your distro, and what version.
Have a read of this.