A response to the "small group questions" for the 6 October 2019 message at First Pres, Boulder. 
Then God said, “Let us make man in our image, after our likeness. And let them have dominion over the fish of the sea and over the birds of the heavens and over the livestock and over all the earth and over every creeping thing that creeps on the earth.”
So God created man in his own image, in the image of God he created him; male and female he created them.
excerpt from the English Standard Version of Genesis 
What is your own best understanding of the doctrine of the Trinity? How do you explain it? What Scriptures, if any, help you?
I scarcely know how to begin with this question.
That's a lie: I know that my fist reaction is, "I can't believe that a message on the Spirit has immediately become about the Trinity in stead." It strikes me as very Presbyterian, that the Spirit is too uncomfortable, too mysterious, to actually talk about for too long, so we have to talk in the abstract about the Trinity in stead.
Erik's message purported to embrace an admittedly detached Presbyterian mindset by defending a discussion about the Spirit before really getting to know the Spirit. This seemed fair enough to me; but then these three things were:
is God (that is, the Holy Spirit is actually God)
is love (but, already, the message went far away from being particularly about the Holy Spirit, in stead saying that "the Trinity", or God in his entirety, is love)
is transformation? (talking about the redemptive work of God in creation; but, again, this was largely talking about God in his entirety, not necessarily or specifically the Holy Spirit)
So I come away from this disappointed that a series purportedly about the Holy Spirit is already in this first message almost entirely about the doctrine of the Trinity, not the least because I consider the doctrine of the Trinity extra-biblical and likely completely incorrect.
The second helvetic confession states that "there are not three gods, but three persons, consubstantial, coeternal and coequal"; but even this simple, essential facet of Trinitarian theology is anti-scriptural.
The prompt asks what scriptures help me with my understanding of the doctrine of the Trinity. I recognize that this isn't the intent; but I can think of no more succinct defense of my leaning than to quote Christ:
You heard me say to you, ‘I am going away, and I will come to you.’ If you loved me, you would have rejoiced, because I am going to the Father, for the Father is greater than I.
excerpt from the New Living Translation of the gospel according to John 
If the Father is greater than Christ, then they are not coequal. And if they are not coequal, there is no Trinity.
I further and specifically reject the assertion that life without the Spirit leaves us with a "two-thirds God" as Erik claimed in his sermon. The Spirit is a comforter to us in our life as inheritors of the kingdom of God; but the Father is not "one third" of God, nor is Christ. Israel was not worshiping one third of God when they worshiped the God of Abraham; and Christ's disciples were not restricted to the presence of one third of God when they followed him.
I have been overly negative here, and I think with purpose; but I am aware that I have failed to answer the question, "What is your own best understanding of the doctrine of the Trinity?" My views on this matter are incomplete, and I am afraid they will remain so until the Spirit has broken my will over the study that will be necessary to complete it. Hopefully that will appear here some day. Not today.
Many protestants work with a functional "Bi-Une" God, focusing primarily on the Father and Son. Why do you suppose that is?
The Father and Son are relatively easy to consider in the abstract, as actors in history or the cosmos relatively distinct from ourselves; but to understand the Spirit is to invite God into your life. To be transformed. "Life," such as we think we know it, without the Spirit is the lie of our age. It is the temptation we are each confronted with: to know good and evil by our own wisdom, rather than to surrender our spirit to God's redeption as his.
What gets lost in our faith and our understanding of God if we set aside the Holy Spirit?
Christ is the promise of an escatalogical salvation from damnation or destruction. The Spirit is the living comfort of salvation from the hell of our broken creation today. In Christ we have hope of life into eternity. In the Spirit the new life starts now.
Throughout this series we will be reminded of the Holy Spirit's vital role in our faith. As we get started, what role does the Holy Spirit have in your life now?
I recognize the Spirit's action in my life, and often. I attibute much of my life and my being to the work of the Spirit, and I consider this attribution an intentional declaration of faith and an act of worship.
John 14:15-31; Romans 8; and Galatians 5:13-26 are some of the prominent places where teaching on the Holy Spirit can be found. What do you notice about the Spirit's work there?
I will ask the Father, and he will give you another Helper, to be with you forever, even the Spirit of truth, whom the world cannot receive, because it neither sees him nor knows him. You know him, for he dwells with you and will be in you.
the Helper, the Holy Spirit, whom the Father will send in my name, he will teach you all things and bring to your remembrance all that I have said to you.
Likewise the Spirit helps us in our weakness. For we do not know what to pray for as we ought, but the Spirit himself intercedes for us with groanings too deep for words.
Many of these passages reflect the role of the Spirit as comforter, as originally promised by Christ to his disciples. I do think that many of the uses of the word spirit (πνεῦμα) in Romans 8 are misattributed and coarsely interpreted to be explicitly the Holy Spirit. Paul explicitly distinguishes the Spirit of God with Πνεῦμα (a capital Π), even though the ESV grossly categorizes all uses of the word as Spirit (with a capital S).
This is notably not the case in Galatians, where most (all?) of the instances of the Spirit are rendered with a capital Π.
How can you seek to be more aware of God as Trinity this week? What may happen if you do this?
Since I'm coming to this three weeks later, I suppose I have missed the call; but, as I expect I've made clear above, I question the validity of the doctrine of the Trinity, so I don't think this call carries much weight to me.
Now if you want to know what might happen if I seek to be more aware of the work of the Spirit this week, that's a whole 'nother deal. I can always use more awareness of God at work in my life, and his presence is felt in my relationships with my family, my community, and even myself. He convicts me when I do not want to do what I ought. He convicts me when I do what I ought not do. And he comforts me with a heart that ever-increasingly desires the things of God.