I will split my answer into multiple parts.
1) Difference einzeln and einzel:
They are the same thing when used as adjectives. einzel is the original form of the adjective and is not commonly used by itself anymore. That being said, there are some commonly used words that use einzel in a combined form. Examples are Einzelgänger (loner, lone wolf), Einzelkind (single child).
2) Difference einzeln and einzelne
They mean the same thing, but einzelne is the feminine version (a single flower - eine einzelne Blume, a single memory - eine einzelne Erinnerung) and also plural if talking about a few individuals of a bigger group (a few students - einzelne Studenten, a few bus stops - einzelne Bushaltestellen).
General meaning: The meaning of all terms is that something/someone is looked as by itself/in isolation. Examples could be something that is physically by itself (a single house up on a hill - einzelnes Haus auf einem Hügel), the order in which something happens (patients are entering the exam room one by one - Patienten tretten einzeln in die Ordination ein), or somthing that happens to a few individuals out of a larger group (A few students have to retake the exam - Einzelne Studenten müssen die Prüfung wiederholen).
adverbial use (no m/f/n):
einzeln - one by one (einzeln eintreten)
einzelner - masculine (einzelner Baum)
einzelne - feminine (einzelne Blume)
einzelnes - neutral (einzelnes Buch)
einzelne Bäume; - Blumen; - Bücher
Hope that helps.