The first one is not a triangle comp, because there's nothing along the lines leading to the other two corners. Second one is because of the water to cloud separation to the left.
Third one is a triangle comp because of the hands and face, and because of the line you left out leading into the distance. Waterfall is an outright trivial example of triangle comp, but I'm disappointed that the water is just water at a slow shutter speed. Staircase is just a weak example, but still a triangle comp. (and I like it, please post larger.)
I've found triangle comp much easier to do than thirds, or spirals.