Prune birch tree October 30
Now is a good time to prune. We would recommend pruning, rather than cutting off the trunk altogether. That would lead to a very big stump, then hole, leaving an opening for rot and disease.
From the photo, it looks like it can easily be pruned up and back to a shape and width that is more to your liking. You can remove up to 1/4th of a tree at a time. Each of these trunks is probably a distinct tree (not 3 trunks of one tree.) Therefore, the 1/4th rule pertains to that trunk, not all of the trunks together.
Here is more info on pruning trees: https://extension.umd.edu/hgic/topics/pruning-trees