List at least THREE reasons some of the African American representatives were determined to be unable to serve​

Hello. Your question is incomplete, which makes it difficult for it to be answered correctly. However, I will try to help you in the best possible way.

If you are referring to African-Americans who were considered unable to serve at the national congress. This happened due to prejudice, the belief that blacks did not have full political knowledge and the attempt by white politicians not to allow the black population to have political representation, as a way to keep the white population superior and dominant.