They defined Trinity as one person as God operating in three modes.
Oneness Pentecostalism - NAMB
I feel that they have misrepresented the Oneness position.
In Jesus all the fulness of the Godhead does indeed dwell bodily (Colossians 2:9). The name of "the son that was given" shall indeed be called "the everlasting Father" because of the zeal of the LORD of hosts (Isaiah 9:6-7).
What I see in the doctrine is that it is not that 1/3 of God descended to become a Man; but that God Himself became a Man; and that Jesus is the Son of God in the incarnation, not that He was eternally begotten. It does not say that "the Son of God would overshadow Mary's womb", but that the Holy Ghost would be the One to conceive of the Son of God in the womb of Mary through the uniting of the Holy Ghost with the egg in the womb of the virgin Mary.
There is one Spirit (Ephesians 4:4); and the Father is that Spirit (John 4:23-24). Jesus is in the Father and the the Father in Him (see John 14:7-11); which indicates that the Spirit of Jesus is the Father. When Jesus died on the Cross, He cried out, "Father, into thy hands I commend my spirit;" and gave up the ghost (Luke 23:46). God could not
vacate eternity when He became a Man.
There is one Lord (Ephesians 4:5, 1 Corinthians 8:6); and the Father is that Lord (Matthew 11:25, Luke 10:21). Nevertheless no one can say that Jesus is the Lord except by the Holy Ghost (1 Corinthians 12:3). And also, the Lord is that Spirit (2 Corinthians 3:17).
The Holy Ghost is that Spirit (John 7:37-39).
This is all speaking of the same Spirit, the same Lord, and the same God (1 Corinthians 12:4-6).
I do not have three Spirits dwelling in me as a believer. Yet the Father dwells in me (Ephesians 4:6); the Son dwells in me (Colossians 1:27) and the Holy Ghost dwells in me (2 Timothy 1:14). Yet there is one Spirit (Ephesians 4:4).
When the Word became flesh and dwelt among us (John 1:14) it should be clear that it was the Holy Ghost who overshadowed Mary's womb (Luke 1:35, Matthew 1:18, Matthew 1:20).
John 1:1 is also interpreted by the understanding that God is outside of time; and that He descended and then ascended once again (Ephesians 4:10) to be outside of time. Thus the same God can exist beside Himself in eternity, i.e. "the Word was with God, and the Word was God;" being the same Person and yet each distinct from the other.