Why does god need to be three(trinity in Christianity)

Im a Muslim , and i always ask fellow christians why does god need to be a trinity [God himself,holy spirit, jesus(word of god)] and they say they are all one and start to say that like water can be in three states but still it is water. But us Humans are only one thing (body,soul,and mind i guess) So why would God need to be divided , which contradicts the believe that hes all powerful (he wont need to separate himself to get things done) ?

and why is the virgin birth of jesus so looked on upon as impossible without a father and not a miracle on one of god's human prophets?

For instance Adam had no father or mother.