[Hattushil]

Okay, let's go step by step...

1- God says in Old Testament,

Thus saith the LORD the King of Israel, and his redeemer the LORD of hosts; I [am] the first, and I [am] the last; and beside me [there is] no God. ....Isaiah - 44:6

and

Fear ye not, neither be afraid: have not I told thee from that time, and have declared [it]? ye [are] even my witnesses. Is there a God beside me? yea, [there is] no God; I know not [any]....Isaiah - 44:8

2- God says in New Testament,

"Do not think that I [Jesus] have come to abolish the Law (the Old Testament) or the Prophets; I have not come to abolish them but to fulfill them. I tell you the truth, until heaven and earth disappear, not the smallest letter, not the least stroke or a pen, will by any means disappear from the Law (the Old Testament) until everything is accomplished. (Matthew 5:17-18)"

"Then Jesus said to the crowds and to his disciples: 'The teachers of the law and the Pharisees sit in Moses' seat. So you must obey them and do everything they tell you. But do not do what they do, for they do not practice what they preach.' (Matthew 23:1-3)"

One of the teachers of the law came and heard them debating. Noticing that Jesus had given them a good answer, he asked him, "Of all the commandments, which is the most important?"
"The most important one," answered Jesus, "is this: 'Hear, O Israel, the Lord our God, the Lord is one.---Mark 12:28 - 30

Thou believest that there is one God; thou doest well: the devils also believe, and tremble.---James - 2:19

Considering the above verses, dear Theja, It is clearly stated in Old Testament that "THERE IS ONLY ONE GOD WHO IS FIRST AND LAST" and "THERE IS NO GOD BESIDE HIM"....So, the God of Old Testament is actually the same God of Quran,

He is the First and the Last and the Ascendant (over all) and the Knower of hidden things, and He is Cognizant of all things. Quran, Chapter: 57, Verse: 003

Oneness of God is clearly mentioned in Old Testament, it is several times mentioned even in New Testament....And Jesus himself clearly states that he came to fulfill the teachings of Old Testament (it also includes the "oneness of God")....It is also several times mentioned in the New Tastement that Jesus did not have any power rather than the power "given" by the God to him and he was all dependant on the "father-the Lord"....With a simple logic,It is clear that everything including "Jesus" and "Holy Spirit" are created by the God...Then,we can safelly say that anything, which is created and dependant, can not be God....This simple logic is applied to Jesus himself and the Holy Spirit....Hence, i would rather prefer to put "Holy Spirit" in his own place as mentioned in Quran and Old Testament,

"Say, the Holy Spirit has brought the Revelation from thy Lord in Truth, in order to strengthen those who believe, and as a Guide and Glad Tidings to Muslims. (The Noble Quran, 16:102)"

"Raised high above ranks (or degrees), (He is) the Lord of the Throne (of Authority): by His Command doth He send the Spirit (of inspiration) to any of His servants he pleases, that it may warn (men) of the Day of Mutual Meeting,- (The Noble Quran, 40:15)"

It is clear that "the Holy Spirit" of Bible is the angel "Gabriel" (or Holy Spirit) of Quran who brought the revelation to the prophets (including Prophet Jesus)...

So, simply Theja, the above looks much more "logic" to me than believing that the god consists of three persons...For time being, i am not going further and just expressing my own logic regarding the matter since i don't want to look attacking your religion and breaking your emotions....

Regards.