These are some ramblings from another perspective that haven't been brought up yet, as far as I can tell.
Jesus was "begotten" by God on the day of His resurrection. Paul testified to this in Acts 13:33-34 when he quoted Psalms 2:7, and interpreted it as God having "begotten" Christ by His bodily resurrection. "God hath fulfilled the same unto us their children in that he hath raised up Jesus again; as it is also written in the second Psalm", (giving an explanation of how that resurrection was the equivalent of God begetting Christ), "Thou art my Son; THIS DAY have I BEGOTTEN thee. And as concerning that he raised him up from the dead, now no more to return to corruption, he said on this wise, I will give you the sure mercies of David."
As for the poetical phrasing of Psalms 110:3, this verse, too, is referring to the day Christ arose from among the dead. More particularly, it is talking specifically about WHAT TIME OF THE DAY Christ arose from the dead; not that he was begotten before the morning star was created during creation week. Christ arose from the dead at sundown, and before the following morning arrived, and after 3 days and 3 nights in the heart of the earth.
I do like the LXX version of this, because it gives a hint of the group of Matthew 27:52-53 "holy saints" that Christ brought with Him out of the grave. "With thee is dominion in the day of thy power" (Remember, Christ said "all power is given unto me in heaven and in earth..." - past tense, even before His Mt. of Olives ascension.) "in the spendours of thy saints", (the Matthew 27:52-53 saints raised with Christ), "I have begotten thee from the womb before the morning." (Jesus as the First-born, First-begotten rose from the dead when the sun went down, before Mary Magdalene ever encountered Him early the next morning on the 1st day of the week.)
This same "power" of a resurrected Christ in Psalms 110:3 is also mentioned in Rev. 12:10-11, after Satan and his angels were cast out of heaven to the earth. It was a power that the resurrected Christ already had when He first ascended to the Father, immediately after He met Mary Magdalene. "And I heard a loud voice saying in heaven, NOW is come salvation, and STRENGTH, and the kingdom of our God, and the POWER of His Christ: for the accuser of our brethren is cast down, which accused them before our God day and night. And they overcame him by the blood of the Lamb and by the word of their testimony." (Meaning that the ascended, first-begotten Christ had just applied His blood on heaven's mercy seat, and had become our high priest intercessor - forever destroying Satan's ability to accuse the brethren before God.)
There are some differences also being expressed above as to whether God or the Spirit raised Christ from the dead. There are verses proving that both were involved. And to add to the mix, we also have this verse John 10:17-18 saying that Christ participated in raising HIMSELF from the dead. So, all three were involved in the process, apparently. "Therefore doth my Father love me, because I lay down my life, that I might take it again. No man taketh it from me, but I lay it down of myself. I have power to lay it down and I have power to take it again." This is the same "power" of Rev. 12:10-11, and Psalms 110:3, and Matthew 28:18, and the "power of His resurrection" that Paul termed it in Phil. 3:10. So, it would seem that the "day of thy power" in Psalms 110:3 was the day the power of Christ's resurrection was on full display.
And the original question in the OP? I find the earliest NT evidence of the Trinity is found in Luke 1:35, within the angel Gabriel's words to Mary. "And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God." At Mary's conception of Christ, we have the Three-in-One present and accounted for in that moment. Unless Gabriel was a liar.