My thoughts of three are the Father, the Son and the Holy Spirit.
I've understood who the Father is since I was a small child attending Indian Creek Baptist Church in Mineral Wells, Texas.
I came to understand who the Son is when I was a young teenager attending The First Baptist Church of Hurst, Texas.
But I didn't really understand (not that I will ever completely understand) who the Holy Spirit is until I participated in a Bible study at First United Methodist Church in Voorheesville, NY. The study guide was Catherine Marshall's book The Helper ... her name for the Holy Spirit.
In reading Christy I learned the meaning of 1 Cor. 13:12 For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known. (emphasis added)
By reading The Helper, I came to understand more about the Holy Spirit ... the Helper ... and the part He plays in the Holy Trinity.
Blessings upon you this day.