How would you write this?

There have been some wonderful stories with amnesiac characters. The movie Memento, for example, is told in a series of backward-running vignettes, because the main character cannot form long-term memories and so only holds onto about 5-10 minutes of reality at a time. It could get gimmicky easily, but I thought the movie was great.

However.

Many, many, aspiring authors (myself included) have FAILED miserably at writing amnesiac characters. Given that you don't have a clear vision for how you want to do this, I would advise scrapping the idea completely. 3 chapters of backstory/infodump/exposition is probably not a good idea.

To be clear, you can make it work if you have a great idea, work hard, and are really talented. But 99% of the time I've seen someone try this (including myself) it has not worked.

I think that could be interesting use of 2nd person.

Try writing it in third, then in second. See which you think works best for the story you're telling.

I think at some point, your MC should start remembering things him/herself. It gets a little dull if it's always someone else who's telling the MC's tale.

You could make it more interesting than just an infodump by a character, maybe they only gain scraps of information at a time, maybe the other person is an amnesiac and they suddenly forget what happened! Something that twists what happens and makes the story different will give you a greater chance of success. But i advise you read little e's post carefully before continuing.

Good luck!

If you did write in first person, I would make sure to include the MC's confusion. For example, say the BFF says she went out with X. MC could mentally be thinking "I did?? I don't remember that. Cool!"

I myself have never written an amnesiac character, but I know what I'd want to see in a book containing such.