A common metaphor is thinking of the sun. The sun has disc (The Father), rays (The Son), and light (The Spirit); all three in one sun.
A more complicated answer:
The differences between The Father, Son and Holy Spirit are as important as the fact that only together they are God. It is false to think of God as one person, to think that only God The Father is God, etc.. The Father is the source of The Trinity and the Will. The Son is begotten by The Father and He is The Wisdom (or Logos, or Icon). The Spirit is proceeded from The Father and He is, well, The Spirit. Each One is one aspect of God, and together They are One. So, there are Three Equal Divine Persons, each fully Divine, but Each has a different quality than the Others.