Aside from the other great answers you’ve received so far, similarity doesn’t prove “copycat.” It’s a statistical principle that correlation doesn’t prove causation.
More importantly, though, the central tenets of Christianity (we’ll say Catholicism here, for clarity) are radically different than the Pagan doppelgangers. While there may have been Pagan tripartite gods, they were essentially different from the doctrine of the Trinity. Even Brahma, the Hindu trinity, is essentially different, whereby Shiva and Vishnu (and all the sub-gods) were “modes” of “forms” of Brahma, which is the heresy of Modalism in Christianity.
And again, Christ as Saviour is essentially different from any Pagan beliefs. He saves us from human sin, whereas all other saviours save us from some external evil.
And again, Christ as a Son of God, or a “God-man” is essentially different from the Pagan counterparts. Christianity professes that Jesus is fully human, as well as fully God. The Pagan counterparts are half human-half god.
And again, resurrection stories in Pagan mythologies are essentially different from the Christian claim. This is for various reasons, but the main one is that Christ’s victory over death has implications for every human person, not so with the Pagan legends.
You can do this sort of thing with every example. Among the different Pagan mythologies, you had similar stories, and they were basically the same content. However, when you arrive at Christianity, many of the stories may appear similar on the surface, but are dramatically different when you understand what each story is about.
Yes, C.S. Lewis did talk about this, and I think the Church does too. All of history points toward Christ. All time before Christ pointed forward to Him. All time after Christ looks back toward Him. Pre-Christ religion prefigured Him. Post-Christ religion attempts to mimic Him, or usurp Him, in some way.