Muslims, Where in the Qu'ran does it define the trinity as Father, Son and Holy Spirit?

No it doesnt. Islam distinctly says that there is only one god with no partners. That verse is simply saying that god asked jesus if he would say to people that he and his mother are godly and to be worshiped. To which he replied that he would never do such a thing as god would be aware of his transgression. Read it carefully then u will understand.