In order to display any specific conditional content on a single post based on it’s category, you have to edit the template called single.php
.
e.g. put something like this in single.php
if ( in_category( 'category1' )) {
//code for category1
} elseif ( in_category( array( 'category2', 'category3' ) )) {
// code for when t
} else {
// code for others
}