In a nutshell, there is one God, eternally existing in three persons: Father, Son (Jesus Christ) and Holy Spirit. The three persons of the Godhead are coequal and co-eternal as stated in Gen.1:26, Isaiah 9:6, Matthew 3:16-17; 28:19
Trinity Doctrine - How Can We Comprehend It? The most difficult thing about the Trinity Doctrine is that there is no way to adequately explain it. The Trinity is a concept that is impossible for any human being to fully understand, let alone explain. God is infinitely higher than we are, therefore we should not expect to be able to fully understand Him. The Bible teaches that the Father is God (Exodus 3:14), that Jesus is God (John 8:58), and that the Holy Spirit is God (Acts 5:3-4). The Bible also teaches that there is only one God (Deuteronomy 6:4; James 2:19). How these two statements of doctrine can both be true is incomprehensible to the human mind. However, this doesn't mean that they're both not true.
Trinity Doctrine - No Illustration Is Completely Accurate With respect to the Trinity Doctrine, none of the popular illustrations are completely accurate descriptions. The water illustration is somewhat the best, but still fails to adequately describe the Trinity. Liquid, vapor and ice are forms of water. The Father, Son and Holy Spirit are not forms of God, each of them is God. Saint Augustine, one of the greatest thinkers of the early church, described the Trinity as comparable to the three parts of an individual human being: mind, spirit, and will. They are three distinct aspects, yet they are inseparable and together constitute one unified human being. So, while these illustrations may give us a picture of the Trinity, the picture is not entirely accurate or complete. An infinite God cannot be fully described by a finite illustration.
About a century later, in 325, the Council of Nicea set out to officially define the relationship of the Son to the Father, in response to the heresy of Arianism which denied the divinity of Christ. Led by bishop Athanasius, the council established the doctrine of the Trinity as orthodoxy and condemned Arius' teaching that Christ was the first creation of God. Each time we celebrate our weekend Mass, we recite the Nicene Creed after the homily, as a profession of not only our belief and faith but also our understanding of who our God truly is for us all – A community of love between three equal yet distinct persons. It is that community of love that we try and emulate in this world.
https://www.allaboutgod.com/trinity-doctrine.htm