All three becoming human is a derivative of the ancient heresy of modalism, the denial of the Trinity, the belief that, instead, God revealed himself in three modes, or forms. The truth is that the son of God did not EVER come into existence, the Son of Man did. Jesus was forever God. He alone took on humanity in order to redeem humanity. The Father did NOT take on a human form. The Holy Spirit did NOT take on a human form.
Besides the irreverent portrayal of God, the crass language spoken throughout the book, the diminished importance of sin, The Shack suggests to the mind of the reader completely unBiblical concepts, a God becoming man in three persons, two females and one male. This is heresy.
"Yea, hath God said ...?"
You can almost hear Satan chuckling as people gobble up this caricature of God that Satan himself has created.
Gender
This isn't something new, the New Agers have long tried to introduce a "god" who is not male. They have even produced a "gender-neutral Bible" that changes all the Scriptures which reference God as "He."
Jesus called him Father. That should settle it.
God calls Jesus Son, but...the god of the shack said: “I am neither male nor female, even though both genders are derived from my nature.” p 93
The book tugs on the emotions to explain it:
“Hasn’t it always been a problem for you to embrace me as your father? And after what you’ve been through, you couldn’t very well handle a father right now, could you?” p 93
So the suggestion is that if you had a bad relationship with your earthly father, God has to be something other than Father so you can accept him. That's heresy. That's giving you permission to remake God into the image you want, thus denying what God has said about himself. It is not for us to make God acceptable to mankind; the purpose of the Gospel is to make US acceptable to God.
God has revealed himself to us as masculine. In his Word he is always addressed in the masculine. The idea that men cannot be comforting or kind, that this characteristic can only come from females is bunk. Furthermore, to change God into a female makes him a goddess, which is not found in Christianity but IS found in pagan religions.
"Yea, hath God said ...?"
Yes, Mr. Young, God DID say!
What follows is an unraveling of Bible doctrine.