Is it permissible for a mature son (25 years old) to shake hands with and kiss his elderly stepmothers (the father's second and third wives) who have been accustomed to shaking hands with him and kissing him since his childhood?

It is not permissible for you to allow your father's wife's sister or your father's wife's mother to kiss or touch you, because they are considered non-mahram (foreign) to you. You should clarify the religious ruling to them, as they are not among the prohibited (mahram) relatives. It is worth noting that some scholars have permitted shaking hands with an elderly woman who is no longer desired.