It's true that most bridges are "two directional."
But three way bridges are pretty rare, globally. I can understand why there wouldn't be many for rivers, but if bridges are designed based on the lie of the surrounding ground, why wouldn't there be a large number of non-river sites that would support such bridges.
On the other hand, three out of the world's bridges exist in Michigan (and only ten or so elsewhere in the United States). What is it about the land, topography, or other features of Michigan that cause it to have a disproportionate number of the country's and world's three way bridges.