From Sunday's material:

Take note that husband and wife are referred to as being one flesh, but are still two distinct individuals. The same reasoning as oneness in three individuals as in Father, Son, and Holy Spirit would also make sense.

John 17 "That THEY may be one even as WE are one"

The THEY is obviously referring to His followers ande the WE is obviously referring to the Godhead, that is also referred to as the Trinity.

The one point that is brushed aside as unimportant is the fact the Father, under the non-trinitarian model, spent eternity in a dark and void universe all by Himself. Not until Jesus came along and began creating things did the Father have others to love and enjoy. The idea that the Father spent eternity alone is too unimaginable to believe.

Yes, quite unimaginable!

Two things are a mystery about God, no, three things.

The begotten Sonship of Christ's eternal pre-existence, his incarnation as the Son of man, and the perfection of his human character of Godliness in our sinful flesh.

Thus, we accept that the Father is the God and Father of Christ, Christ is the Son of God and equal to him, as announced to the heavenly host before Lucifer fell, and we receive the Holy Spirit from them now that Christ is ascended to heaven - not guarranteed when he was incarnated, for he took infinite risk of losing his divinity when becoming a man should he sin - that we may live for God through Christ Jesus our Lord.

There is, of course, much good in this lesson: it speaks of God saving us from sin, in the person of his Son.

This bit, though: "...God is One and composed of three "Persons"", is a problem. "They" is allowed for the persons of the trinity, under further reading; yet, they are parts of a divine Being, the Trinity.
This says it well: personal distinctions are "the way God is in himself". That isn't just difficult to grasp, it's not a personal God, is it, God and Christ, both present within by their Spirit? "God, Christ, and the Holy Spirit" are "God...in himself"? Please help me differentiate that from modalism?? It may be meant to be miles from modalism - as is said at the end of the lesson, but it's the language of modalism, or? How the Nicene Creed, with its formless substance of orthodox teaching, avoids modalism is a technicality, in their thinking, perhaps: the "personae" are "centres of consciousness" in that substance, rendering them distinct.

As the Bible reveals the living God, the members of the Godhead have identities much more personal and real than that. God and his Son are spoken of as is the Spirit of God: how significant is the identity of Christ as the Son of God, in the Bible? Is it just his incarnated relationship to his Father or also the very basis of his deity in his pre-existence, as the Pharisees recognised, too, making him equal with God?

The first sentence, here, is not in the printed quarterly, btw. check here, http://absg.adventist.org

It is similar to the first question at the end of the lesson, but is rather more strident.

It's historical reference is simply wrong: our early church's teaching was firmly and decidedly against the doctrine. Our beliefs, on the other hand, shouldn't really be "unrelenting", for the Bible is our rule of faith.

Indeed, Father, Son and Holy Spirit are separate persons each possessing one Godhead, but "echad" isn't exclusively about a complex unity: ask any Jewish Rabbi, it means numerically one, too. There's no linguistic significance at all in "echad" denoting plurality, sorry. Even in the Bible itself, this Hebrew word is used in a mathematical "singular" meaning many times.

I personally think that a lot of confusion comes from our use of "Father & Son". God is not the Father of Jesus Christ.

What do you mean? - remember this?