Invite (all) to the way of thy Lord with wisdom and beautiful preaching; and argue with them in ways that are best and most gracious: for thy Lord knoweth, best who have strayed from His Path, and who receive guidance. (Qur'an 16:125)
A person can also be a father, a brother and a businessman at the same time, but yet, he is one and the same person. Then why cannot God be "one in three" i.e. God the Father, the Son and the Holy Ghost?
The same person can be a father, a brother and a businessman at the same time.
a.But suppose the sister of that man tells him a secret, it is but natural that the father and the businessman also will know about it. Contrary to this, Jesus (pbuh) has said in the Bible, in the Gospel of Mark, chapter 13 verse 32
"But of that day and that hour knoweth no man, no, not the angels which are in heaven, neither the Son, but the Father."
Here the knowledge of the hour is only known to the Father and not to the Son, indicating that God and Jesus (pbuh) Christ are not one and the same person.
b.A human being can be a father, a brother, and a businessman at the same time, but if the brother dies, even the father and the businessman will die. Thus, when the Christians say that Jesus (pbuh) died on the cross they will have to agree that God and the Holy Spirit also died.