IIlicit means something is forbidden by law, custom or regulatory rules. Therefore, an action, which is considered to be illicit is one, which breaks a law, such as speeding. Or an action one does or refuses to do in compliance with a custom. Wearing blue jeans and a tee shirt to a formal dinner is ignoring the custom of appropriate dress for formal occasions. Organizations have rules, which govern the behavior of the members and when someone breaks one of the rules, it is referred to as an illicit behavior. Behavior outside the rule, law or custom.

IIlegitimate means a conclusion is not logical, therefore it would be considered an illegitimate conclusion as the conclusion of a debate or argument. This word also refers to the offspring/children, of a man and woman not legally married to each other. This word also means an action, which is contrary to law or rule or not keeping with accepted usage, such as incorrect use of words and phrases.

Illegal means actions or behavior, which is in defiance of a
law. It can also means actions, which are not approved, sanctioned or otherwise authorized by law. Examples would be to ignore signs and statements, which clearly warn us about a law: Unlawful to enter beyond this point; Speed Limit 70; Adults Only, or This information is protected by copyright laws. If, we ignore this information then our actions would be illegal actions.