Throughout history there have been many attempts made to understand the Trinity. Some of these attempts were good and helped us advance our knowledge of the divine, but some fell to heresy in an effort to explain. Yes, the Trinity is a plurality of persons, but this does not contradict divine Oneness. The three persons of the Trinity have different operations as it is the Father who creates, the Son redeems, and the Holy Spirit sanctifies (CCC 235). Some try to explain this away with the heresy of Modalism which states that God operates under different modes, or manifestations, throughout history.
We can look into the pages of sacred scripture to show the plurality and Oneness on full display. At the baptism of Christ, we read of the newly baptized Christ, the voice of the Father speaking from heaven, and the Holy Spirit descending like a dove (Matthew 3:13-17). Neither is the Trinity three gods, but tradition from the earliest days of Christianity, and scripture, teach Oneness. These heresies popped up to understand, and in some cases, to undermine divine revelation regarding the Trinity. As previously stated, the persons of the Trinity are united in purpose, have functions, and the ultimate goal of our redemption. They are the same substance, and as such each is wholly and truly God.