The invention comprises a triform fitting 1, essentially made from non-metallic composite materials including several strengthening cross ribs 4 for joining symmetrical flanges 3 and a longitudinal flange 2. The symmetrical flanges 3 include cross slots 8 for receiving the run-out 16 of the webs 11 of several T-shaped internal stringers 9 that form part of the torsion boxes 6. The feet 10 of the stringers 9 are joined along one of their faces to the skins 7 of the torsion boxes 6, while the end zones of the free faces of said feet 10 are placed against the outer face of the symmetrical flanges 3 of the triform fitting 1. This ensures that the symmetrical flanges 3 of the triform fitting 1 remain inside the torsion boxes 6. Alternatively, the fitting 1 may not include the cross slots 8.