1.Broadcast address is relative to a network, so every n/w will have it's own broadcast adress(I can obtain the broadcast address, by
replacing all the n/w bits to 1)
Not quite, for any given subnet, the Broadcast Address is found by taking a hosts IP address, and seting any bit positions which hold a '0' in the subnet mask to '1'.
For example, the broadcast address of 192.168.1.1/24 is 192.168.1.255 because the subnet mask defines the host bits as the last octect, and if they are all changed to 1 in binary, the end result is 11111111 or 255.
Since a "Network" may refer to an autonomous system larger than a subnet, the beginning of your question might be better phrased as as a "A Broadcast Address is relative to the subnet on which it originates...".
2.When the destination IP address is a broadcast address,the router will send out the packet(by replicating it) out of all available
interfaces. That means, if a router has interface f0/1 , f0/2 and f0/3
--then 3 packets will be sent out(each being sent out of one of the interfaces)
Not exactly, you're describing how an IP packet is forwarded out physical interfaces, but without describing the IP Adresses of those interfaces, there is not enough information to determine how a broadcast packet would be forwarded.
For example, the router may have different subnets available to it on each one of those fast ethernet interfaces, and each subnet has it's own broadcast domain.
3.A frame with destination layer 2 broadcast address ( FF:FF:FF:FF:FF:FF) is sent to all the devices in the SAME LAN
That is correct, and it is worth noting that this is how an ARP request works. If an IP Address needs to be resolved into a MAC address, a broadcast is sent out to all the hosts requesting a reply from the host which has the correct IP address. The host responds in the affirmative with it's MAC address, and in that way the IP is resolved to the MAC.