Surah 5:72. They do blaspheme who say: “(Allah) is Christ the son of Mary.” But said Christ: “O Children of Israel! worship Allah, my Lord and your Lord.” Whoever joins other gods with Allah,- Allah will forbid him the garden, and the Fire will be his abode. There will for the wrong-doers be no one to help.
73. They do blaspheme who say: Allah is one of three in a Trinity: for there is no god except One Allah. If they desist not from their word (of blasphemy), verily a grievous penalty will befall the blasphemers among them.
Actually, the doctrine of trinity that is denied in the Quran is NOT the doctrine of trinity that Christians believe in. The doctrine of trinity in the Quran is about three gods. The Christian doctrine of trinity is about one God is three Persons.