Technically, these are considered three "aspects" of the one God. When the early Christians decided that Jesus had to be divine, they had to find a way to make him so without actually devolving into polytheism, so they developed this Trinity idea. Yeah, it sounds silly. You have to take it on faith!
Many other things which supposedly made Jesus unique were also relatively common among pagan religions at the time or before Christianity, including the virgin birth. This is one of the reasons why religious scholars are questioning the existence of Jesus as a real person.Many pagan religions have sacrifice and resurrection in their beliefs, long before Chrisitanity. It is a well established idea.
The resurrection stories of the pagans were very common before the time of Jesus, too. They all derive from the rebirth of spring after the death of winter.In my own world, this refers to what dies so we can live, and then grows again or is born next year. Nature's cycle, in other words.