The biblical prophets prepared the way for the promised Messiah. We all know there is only One God and as Jews and Christians had been worshipping the same ONE God, I'm not sure why Islam felt the need to reinforce a belief that was already known and practiced.

In regards to the trinity, which trying to explain it to a Muslim is a pointless exercise, so let's not bother. :) the trinity is a concept in the way tawheed is a concept, the word is not found in the Bible as the word tawheed is not found in the Quran. However the concept for both can be found throughout the respective books. Where you choose your Bible verses that you believe prove the "oneness" of God I could just as easily choose verses that point to the triune concept of God. To what aim?

The he sticking point for Christians and Jews with Islam, is not the concept of One God, because we already accept that, it is the acceptance of Mohammed as prophet and the final messenger to all mankind. Mohammed does not fit the Jews requirement for their Messiah, who they still wait for. Christians do not accept Mohammed because Jesus said with Him it was finished, and so were not expecting anyone else. Jesus is all we need.

peace.