Method of manufacturing a metal cord with three concentric layers (C1, C2, C3), of the type rubberized in situ, i.e. during its manufacture comprising a first, internal, layer or core (C1), around which there are wound together in a helix, at a pitch p2, in a second, intermediate, layer (C2), N wires of diameter d2, N varying from 3 to 12, around which second layer there are wound together as a helix at a pitch p3, in a third, outer, layer (C3), P wires of diameter d3, P varying from 8 to 20, the said method comprising the following steps: a sheathing step in which the core (C1) is sheathed with a rubber composition named “filling rubber”, in the uncrosslinked state; an assembling step by twisting the N wires of the second layer (C2) around the core (C1) thus sheathed in order to form, at a point named the “assembling point”, an intermediate cord named a “core strand” (C1+C2); an assembling step in which the P wires of the third layer (C3) are twisted around the core strand (C1+C2); a final twist-balancing step.