I never have believed the Vatican’s official bit about the 3rd message.
And, all the messages sound to me like they are quite geared to intensifying respect for, adoration of, submission to the caricatured pseudo-Mary personage the Vatican has elevated into sharing the “hypostatic union” with The Trinity.
Quite a clever thing hatched in the pit.
Born and raised Catholic, (not so much anymore), I never did buy the church's reasoning of not revealing the third secret when told to do so. If the pope, at that time, really believed that it was the virgin, who was he or any mortal for that matter not to follow her orders to the letter.
Something happened I have no doubt, but what really happened is the question.
The "hypostatic union" refers to the union of Christ's human and divine natures in one Divine Person. It has nothing to do with Mary, except that she helped to bring it about.
I don't know what you think "Mary sharing the 'hypostatic union' with the Trinity" means (the phrase makes no theological sense), but it's not Catholic doctrine.