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."