My own conception of a timeline is of a one-dimensional timeline, i.e., events placed somewhere on a stright line.
Now two-dimensional implies two variables: time and another one (could be place, or temperature, etc).
Three-dimensional thus needs three variables, and it wouldn't be called a timeline, as would also be the two-dimensionnal one: they would rather be 2D ans 3D graphs.
But I'm just a college student, maybe there is another answer...