They are all valid, in different contexts. My friend was unlucky, not successful in something - I was disappointed about my long time friend (i was disappointed about their misfortune) My friend behaved in a poor way. I was disappointed in/with my longtime friend I was expecting something to occur, but due to the action of my friend, it did not occur. I was disappointed by my long time friend

In or With are the best choices here I think.

I suspect that the writer of this sentence expects you to choose 'by'. But in fact, you could use any of these prepositions in this sentence. The meaning between them would be slightly different, but they are all possible.

Thank you )

According to the Cambridge Advanced Learner's Dictionary,the correct use are:be disappointed at/about sth.||be disappointed in\with sb. Therefore,the right one is that I was disappointed in\with my longtime friend. Hope that helps.