I like John Frame’s concise explanation on why it is unorthodox,
Some have thought to reconcile these positions by saying that God is really one but only apparently three. Father, Son, and Spirit are just the one God playing three different roles. Sometimes he appears as Father, sometimes as Son, sometimes as Spirit. This position is called modalism, since it makes the three persons only modes, or ways in which God exists – not real persons.
The church rejected modalism as a heresy. It is clearly unbiblical, for the three persons enter transactions with one another. Jesus prays to the Father (John 17); the Father speaks from heaven while Jesus is on earth (Matt. 3:17). The Father and Son together send the Spirit into the world (John 14:16, and the Spirit bears witness to Jesus. The Spirit is “another” Comforter, not the same as Jesus. The three glorify and honor one another. Here we see three different persons, interacting with one another, conversing as human beings do, not just one person playing three roles.
In redemption, the Father foreknows, the Son sprinkles blood, and the Spirit sanctifies (I Peter 1:2). To generalize, the Father foreordains, the Son accomplishes, and the Spirit applies the work of Christ to the heart.
Aren’t you glad the Triune Godhead is one God in three distinct persons, not just one God that switches out between three different masks? It is mysterious, but the distinction of the Triune Godhead is a wonderful thing! Let us walk by faith, believing what the Bible teaches us about this glorious doctrine!