The idea of the Trinity is that God is one in essence (or being or nature) and three in person, each of the three persons fully and eternally sharing in the one divine essence.
This would be a logical contradiction if it were affirming that God is one and three in the same way at the same time. However, the affirmation is that God is one in one aspect (essence) and three in a different aspect (person).
Christians have maintained that God is a Trinity (tri – plural, unity – one) since the first century. Contrary to what some think, the Trinity is not an intriguing yet impractical theological oddity; rather, it is absolutely integral to the Christian faith.