1 John 5:7 says, 'For there are three that bear witness in heaven: the Father, the Word, and the Holy Spirit; and these three are one.'
We would explain it this way:
1. God is one (Deuteronomy 6:4). Therefore, whatever explanation we come up with cannot contradict that essential reality.
2. For us there is one God, the Father, of whom are all things, and we for Him; and one Lord Jesus Christ, through whom are all things, and through whom we live (1 Corinthians 8:6). The one God is the Father. The one Lord Jesus Christ is God in the flesh, the Father in human form. This is not describing two divine persons, but rather God as eternal Spirit (the Father) and God in flesh (the Son).
3. God the Father exists as both Word and Spirit. The passage from John is not saying that the Father, the Word, and the Spirit are three distinct divine persons. It clearly states: 'These three are one.' In other words, we must not divide God from His Word or from His Spirit. The three terms are simply different ways of describing the one God.
What we are saying is that when you attempt to explain a verse such as this to someone, use it as an opportunity to explain the larger idea of the Oneness of God. Don't try to explain it as an isolated verse.