Do you mean as standalone sentences? If so, these three pairs mean very much the same thing, but the versions with "on" can seem more chatty: Come in and Come on in Come out and Come on out Come back and Come on back If you are asking about arbitrary sentences starting with these words, the versions with "on" will probably be usable with more or less the same meaning, but again a more chatty feel, provided the sense is right. You cannot make arbitrary substutions without considering the sense.
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