The Legend of the Christmas Tree Board Book Book

I'm not sure if the story told in this book about the legend of the Christmas is true or not, because from what I've learned before, the tradition of decorating the Christmas tree started from the time of King George III and Queen Charlotte of Great Britain, and was popularized by Queen Victoria. Maybe the tradition dates back longer than that, or maybe not. Either way, I didn't find the story in this book as touching, memorable or uplifting as other stories in this Christmas legend stories.

This book also said that a tree has three corners that resemble three persons that God exists in. But what does "a tree has three corners" mean? What kind of corner that a tree has? Someone please enlighten me, because from what I see, a tree has no corners, and trees have nothing to do with God's persons...