The current definition of 3D digital lines, which uses the 2D digital lines of closest integer points (Bresenham's lines) of two projections, has several drawbacks: the discrete topology of this 3D digital line notion is not clear; its third projection is, generally, not the closest set of points of the third euclidean projection; if we consider a family of parallel euclidean lines, we do not know how many combinatorially distinct digital structures will be built by this process; and mainly the set of voxels defined in this way is not the set of closest points of the given euclidean line. And these questions are the simplest ones; many others could be asked: dependence on the choice of the projections, intersections with digital planes, intersections between 3D digital lines, ... . This paper gives a new definition of 3D digital lines relying on subgroups of Z3, whose main advantage over the former one is its ability to convert any practical question into rigorous algebraic terms. It follows previously developed ideas but with a much simpler treatment and new results. In particular, we obtain a complete description of the topology of these lines and a condition for the third projection being a 2D digital line as well as a classification of digital lines of the same direction into classes of equivalent combinatorial structure