Tensioner for the gripping and the controlled forward movement of an elongate element, in particular for winding/unwinding a cable, a hose, a pipeline or an umbilical on a reception member, wherein said tensioner comprises at least one frame that carries three guide boxes arranged around a passage axis (P'): - a central guide box, which extends over a central plane passing through said passage axis (P'), - two lateral guide boxes, which are arranged symmetrically with respect to said central plane, wherein said lateral guide boxes are each arranged in a plane defining an angle of between 115° and 125° with respect to said central plane, wherein said guide boxes each comprise a support body on which a guide chain is guided, which is driven by at least one motorized wheel, wherein said guide chains each comprise a front strand, intended to provide a contact line on said elongate element, which extends parallel to and opposite said passage axis (P'), said guide boxes cooperating with said frame by means of maneuvering means (5, 6) and said tensioner comprises control means for controlling a synchronous conveying in the same direction of said front strands and for adjusting an equidistant separation of said front strands with respect to said passage axis (P'), wherein said maneuvering means (5, 6) comprise: (i) first maneuvering means for the maneuvering of said central guide box in rectilinear translation, in a direction oriented in said central plane and perpendicular to said passage axis (P'), and (ii) second maneuvering means, comprising: - support means comprising at least one rocker means (71, 72) supporting said lateral guide box associated, wherein said at least one rocker means (71, 72) comprises a proximal end (711, 722) mounted so as to pivot on said frame and a distal end (712, 722) mounted so as to pivot on said lateral guide box, respectively about rotation axes (711', 712'; 721', 722') lying parallel to said passage axis (P'), wherein said proximal end (711, 722) supports said lateral guide box associated, and - actuator means for maneuvering said support means and said associated lateral guide box, for maneuvering each of said lateral guide boxes in rectilinear, or at least substantially rectilinear, translation, respectively in a direction defining an angle of between 115° and 125° with respect to said central plane, characterized in that, at each lateral guide box, the support means comprise two rocker elements: - an external rocker element, and - an internal rocker element, provided between said external rocker element and the passage axis (P'), in that said rocker elements (71, 72) have a distance between centers corresponding to the distance between their respective rotation axes (711', 712'; 721', 722'), and in that said external rocker element and said internal rocker element each comprise a distance between centers wherein the length is fixed. - a central guide box, which extends over a central plane passing through said passage axis (P'), - two lateral guide boxes, which are arranged symmetrically with respect to said central plane, wherein said lateral guide boxes are each arranged in a plane defining an angle of between 115° and 125° with respect to said central plane, (i) first maneuvering means for the maneuvering of said central guide box in rectilinear translation, in a direction oriented in said central plane and perpendicular to said passage axis (P'), and (ii) second maneuvering means, comprising: - support means comprising at least one rocker means (71, 72) supporting said lateral guide box associated, wherein said at least one rocker means (71, 72) comprises a proximal end (711, 722) mounted so as to pivot on said frame and a distal end (712, 722) mounted so as to pivot on said lateral guide box, respectively about rotation axes (711', 712'; 721', 722') lying parallel to said passage axis (P'), wherein said proximal end (711, 722) supports said lateral guide box associated, and - actuator means for maneuvering said support means and said associated lateral guide box, - support means comprising at least one rocker means (71, 72) supporting said lateral guide box associated, wherein said at least one rocker means (71, 72) comprises a proximal end (711, 722) mounted so as to pivot on said frame and a distal end (712, 722) mounted so as to pivot on said lateral guide box, respectively about rotation axes (711', 712'; 721', 722') lying parallel to said passage axis (P'), wherein said proximal end (711, 722) supports said lateral guide box associated, and - actuator means for maneuvering said support means and said associated lateral guide box, - an external rocker element, and - an internal rocker element, provided between said external rocker element and the passage axis (P'),