These three occasions, when a voice spoke to Jesus from the sky, mark stages in the Gospel story when Jesus understood and accepted the destination of his ministry. He wasn’t was born knowing the full script for his life. He was born as a normal, human baby lying helpless in a cattle trough – then he learned his destiny by stages.
Years ago, I was taught that the Transfiguration was the fulfilment of Jesus' promise that some disciples would see him "coming into his kingdom". I felt dissatisfied with that explanation - but now I see the point.