Blender question, creating edge vertices

I have a problem. Creating simple objects in Blender (really simple, like basic shapes)

I know that if i have a cube for example. Since each vertex is part of 3 triangles there has to be 3 vertexes at the same point with 3 different normals or you dont have a sharp edge. I dont know how to make the 3 vertex thing happen in Blender though, just how to get it by telling unity to calculate normals.