The first two talk about an imagined past. I can't explain much more, because (as Michael has also explained), you need to know a certain amount of English grammar before you can understand those examples. Do the work, and then it will become clear. :)

You've asked about three different grammatical structures which are used to express different things. It won't really help you if we give you a conceptual explanation of their differences. I recommend that you study standard course book material on the past perfect and the main conditional forms. Once you have done this, you will be able to ask more specific questions in order to clarify any difficulties.