Muhammad was not a very well informed theologian when he wrote the Koran.

Mohammed mistakenly thought that Mary was the third member of the Christian trinity.

"O People of the Book! Commit no excesses in your religion: Nor say of God aught but the truth. Christ Jesus the son of Mary was (no more than) an apostle of God, and His Word, which He bestowed on Mary, and a spirit proceeding from Him: so believe in God and His apostles. Say not "Trinity" : desist: it will be better for you: for God is one God: Glory be to Him: (far exalted is He) above having a son. To Him belong all things in the heavens and on earth. And enough is God as a Disposer of affairs." (Koran 4:171)
[link to www.bible.ca]