Why is Trinity Necessary After Jesus' Death? Can't God Exist As One?

Why can't God exist as One when there's no point of Him existing in 3 forms that too after death of Jesus. Why is Trinity necessary as it gets confusing everytime you try to pray to God. Furthermore, there are many questions (Look at them ) than answers when it comes to Jesus calling God as one at many places in the Bible. My question is if God can manifest in 3 forms, there is a higher chance of him existing in more than 3 forms? Isn't it. Why stop at 3? So it would have been much better if God existed as one in all respects for there would be no contradictions. If God is Sufficient in all Respects, then What's the need of Holy Spirit Or Jesus to exist. What's their role in running the affairs of this Universe. Ain't God as one, sufficient?