A bit aboug flashbacks. A look into the past can be just a line of memory, a passage of exposition, a conversation or a genuine flashback that is a scene that has taken place in the past. The first kinds of looking at the past seldom cause problem, but the long scenic flashback can pull the reader out of the story. One of my favorite writers used this in one of her books where perhaps a chapter or two were in the present and then one in the past. I'm sure she was just trying a technique but for me the story was so disjointed I began to skip the past scenes and focused on the current story. When the information is vital, try to find another way of presenting the information. Used judiciously a memory of the past can hold clues to the present problems in the story.
Prologues and epilogues are generally better left off. For the prologue if there is information that is vital to the story use a prologue but remember to make it vital and short. If the material in the prologue goes on for pages and pages, this may mean the story has started in the wrong place.
Epilogues tell something that happens in the character's futures. I've never been partial to them. This is my own view but often they're found in romance novels and make me feel the writer is trying to convince me that this romance really works. Something to think about. If a writer is planning a sequel to the book, an epilogue can act as a teaser for what is to come.
Remember to use these three additions to the plot judiciously. Keeping the reader turning the pages is a good thing. Having something that causes them to stop reading isn't good.