"Just" is used as an adverb (to describe an adjective or a verb) and it has the same meaning as "only" in that context. For example: She was only twelve when she won her first national science fair. In this sentence "only" can be replaced by "just" and the sentence will still have the same meaning. "Only" can also be used as an adjective, in which case it means that there is one of whatever it is you're talking about. For example: This is the only key to the door. "Alone" means that an object is by itself. (She was alone in the room.) You can change the way these sentences are written to use any of the above words. So instead of saying "She was alone in the room" you could also say "It was just her in the room" or "She was the only one in the room" I hope this wasn't too confusing! (Note, the above is my attempt at condensing the following link, which explains the difference between "just" and "only" much better than I do: http://www.ecenglish.com/learnenglish/lessons/only-and-just I don't really have any links explaining the difference between all three of them, though).

I think you mean "What is the difference between 'just', 'only', and 'alone'?" Another word that means the same thing as "just" is "simply." You might use this when talking about something that is easy. For example, "It is not difficult to get to the store, you just walk down the street." Alone refers to a single noun by itself. For example, "She walked alone to the store" or "we left the dog alone at home." Only refers to how a noun is different than other things. For example "there is only one white egg, the rest of them are brown" or "I can only drive to work, I cannot walk."