A device and method for manufacturing a metal cord with three concentric layers, rubberized in situ, of M+N+P construction, wherein the method comprises the following steps which are performed in line: an assembling step by twisting N wires around a first layer to form, at a point named the “assembling point”, an intermediate cord named a “core strand” of M+N construction; downstream of the assembling point, a sheathing step in which the M+N core strand is sheathed with a rubber composition named “filling rubber” in the uncrosslinked state, an assembling step in which P wires of the first layer are twisted around the core strand thus sheathed, and a final twist-balancing step.