So I have heard that at least 3 Christian festivals have roots in paganism:
Halloween
Christmas
Easter
What you heard was misinformation that a lot of people are quick to believe and then repeat to others as if it were the gospel. As
@Rival pointed out, the Christian world beyond the Anglosphere does not call the festival Easter but Pascha, which actually links the festival to
Pesach or Passover. Also, if you look at some of the writings of the Church Fathers, Christians in the Mediterranean and the Middle East were calling it Pascha before anybody called it Easter. Moreover, on Pascha, Christians celebrate the central event in the life of Jesus
as narrated in the canonical Gospels, not as narrated in a pagan tradition, which would not even involve Jesus.
As to Christmas, I remember reading about how Eastern Christians at first observed what they call the Nativity of the Lord on different dates depending on their geographical locations. Some of the dates were in December whereas others were in January. Today, most Eastern Christians observe it on 25 December according to the Julian Calendar and the Revised Julian Calendar. The Armenian Apostolic Church observes it in January according to the Gregorian Calendar, which most Armenian Churches use. Considering that Orthodox, Catholic, and classical Protestant Christians have a high view of the humanity of Jesus, it is no surprise that they celebrate his nativity on any day of the year.
The ancient Egyptians celebrated the birthdays of Aset, Wesir, Nebet-Hut, and Sutekh. Millions of Hindus to this day celebrate the birth of Kṛṣṇa. Does this suggest
Kṛṣṇa Janmāṣṭamī is rooted in ancient Egyptian religion or that the ancient Egyptians' celebrations are rooted in Hinduism? No. On the other hand, if Christians celebrate
any event that occurred in the life of Jesus, it
must be rooted in paganism according to some people. It's really stupid. Speaking of stupid, the same sort of people claim the Ka'bah in Makkah was originally a Śiva temple.
Due to this are some denominations of Christianity more authentic than others?
In my opinion, how authentic any denomination or Church is probably depends on its origin.