The leg sections of the current crop of carbon fibre Gitzo tripods are made in various thicknesses and diameters.
The tube diameters used are 41.3 mm, 37.0 mm, 32.9 mm, 29.0 mm, 25.3 mm, 21.7 mm, 18.2 mm, 14.7 mm and 11.2 mm.
The top leg sections of a "series 5" are 41.3 mm and those of a "series 0" 21.7 mm, so a three-section "series 3" would have top leg diameters of 32.9 mm and bottom leg diameters of 25.3 mm.
For a given material the stiffness of a tube is proportional to the wall thickness and the cube of its diameter, so the 25.3 mm bottom leg sections of the three-section "series 3" would be 2.7 times as stiff as the 18.2 mm bottom leg sections of a four section "series 2". The weight difference though, would only be about 200 g.
John