As i understand in distributed system we are supposed to handle network partition failure which is solved by using multiple copies of the same data.
Is this the only place where we use consensus algorithm? What is the difference between 2PC/3PC/Paxos (is paxos modified version of 3PC? if so then 2PC/3PC , PC also kind of consensus algorithm?)
Network partition is not "solved" by having many copies of the same data. Although redundancy is ofcourse essential to deal with any sort of failures :)
There are many other problems regarding network partition. Generally to increase tolerance for network partitions you use an algorithm that relies on a quorum rather than total-communication, in the quroum approach you can still make progress on one side of the partition as long as f+1 nodes out of 2f are reachable. Paxos for example uses quorum approach. It is quite clear that a protocol like 2PC cannot make progress in case of any type of network partition since it requires "votes" from all of the nodes.
What is the difference between 2PC/3PC/Paxos (is paxos modified version of 3PC? if so then 2PC/3PC , PC also kind of consensus algorithm?)
2PC/3PC/Paxos are all variants of consensus-protocols, although 2PC and 3PC are often described as handling the more specific scenario of "atomic commit in a distributed system" which essentially is a consensus problem. 2PC, 3PC, Paxos are similar but different. You can easily find detailed information about each algorithm on the web.
Is this the only place where we use consensus algorithm?
Consensus protocols have many many use cases in distributed systems, for example: atomic commit, atomic broadcast, leader election or basicly any algorithm that require a set of processes to agree upon some value or action.
Caveat: consensus protocols and related problems of distributed systems are not trivial and you'll need to do some reading to get a deep understanding. If you are comfortable reading academic papers you can find most of the famous ones available online, for example "Paxos made simple" by Leslie Lamport or you can find good blogposts by googling. Also the wiki-article for paxos is very good quality in my opinion!
Hope that could answer some of your questions although I probably introduced you to even more! (if your interested you have some research to do).