Salam alaikum. May Allah bless you and your family.
The fundamental basis of Islam is the concept of tawheed which refers to to the absolute unity and oneness of God. Although Christianity believes in monotheism, they believe that God - although one in nature - has three persons, namely the Father, the Son and the Holy Spirit. According to Islamic doctrine, God or Allah is not only one essence but He is also one person, not three. I hope this answers your question.
To learn more about tawheed (tawhīd), please see our following lessons on the site:
As-Salamu Alaikum Wa-Rahmatullahi Wa-Barakatuh