Sessions resigning instead of being fired allows for POTUS to appoint a temp replacement instead of the position defaulting to the deputy AG. I can't say anything definitive about the other two other (I don't remember what positions they resigned from at the moment) than it wouldn't reflect negatively on their ability to be hired in the future. Being 'fired' really destroys your career, as it implies some sort of poor conduct on your part whereas resigning is relatively neutral.