The Trinity and Mary

[Fake Messiah]

How do you know that they took the gospel accounts from these stories?

What do you mean "how do I know"? The elements were already there and you see Justin Martyr saying it. And when it comes to Jesus moral teaching we already had discussion in the topic "Love thy neighbor" on the 5th page that all the so called sentiments attributed to Jesus are were familiar in the Jewish schools, and to all the Pharisees, long before the time of Christ, as they were familiar in all the civilizations of the that time - Egyptian, Babylonian, and Persian, Greek and Hindu.

Also even many theologians now admit that it is not likely that we have the actual words of Christ at all. One example in Matthew 11:28-30: “Come to me, all you who are weary and burdened, and I will give you rest. Take my yoke upon you and learn from me, for I am gentle and humble in heart, and you will find rest for your souls. For my yoke is easy and my burden is light.”
We may say with confidence that Christ never spoke those words. The doctrinal implication of divinity is late: the words are inconsistent with his constant insistence that his burden is heavy and grievous (to sell all one's property, to separate from one's family, to take up one's cross, etc.); and, most decisive of all, the words are simply borrowed, with little alteration, from Ecclesiasticus (51, 23-27). It is, therefore, extremely difficult to say anywhere that we have the actual words of Christ and it is quite certain that many of the passages we cannot attribute to him are as fine and impressive as the doubtful passages.

You just took three gods from each culture and made "trinities" out of them.

I didn't took them. When it comes to Roman Capitoline Triad the Romans took them.
https://en.wikipedia.org/wiki/Capitoline_Triad