To believe in more than one God opens the door to inconsistent first causation and other questions. If there was more than one god, does that mean that there was an original / more superior god that made the others? If so, wouldnt worshiping to one of the secondary gods make the superior god jealous and be misplaced since he made the god being worshiped?
See the sections About Jesus and About the Holy Spirit to see how they both have the characteristics of God, are co-eternal, and distinct but still One, aka The Trinity. All human analogies to describe The Trinity fall apart but the closest is we are told to pray to God the Father, through the mediator and authority of Jesus the Son, and lead by the Holy Spirit God what to pray. Or God chose before the foundation of the world who would be saved, Jesus lived the perfect life, paid the penalty of sin and conquered death for those that would be saved, and the Holy Spirit draws, convicts, enables those that would be saved to repent. The praise and glory for both prayer and salvation go to one God!