Background <p>S³ of the left upper lobe (LUL), situated between S^1 + 2 and S^4 + 5, serves as a vital anatomical bridge. This study examines the anatomical relationships of S³ with surrounding segments to enhance surgical accuracy and minimize complications.</p> Methods <p>A total of 120 patients with LUL lesions scheduled for surgery from October 2024 to March 2025 were included. Three-dimensional CT bronchography and angiography (3D-CTBA) allowed us to analyze S3 anatomy, its spatial relationships with S^1 + 2 and S^4 + 5, and lesion locations. Trans-segmental and trans-subsegmental arteries were defined.</p> Results <p>S³-related lesions accounted for 46% (61/133) of all LUL lesions. B³ branching was classified into three types, with B³a; B³b + c being the most common (86%, 103/120). An independent origin of B^1 + 2c limited S³a to the central region of the LUL, while the two-branch LUL bronchus and semi-central vein were associated with S³a extension to the oblique fissure (<i>p</i> = 0.029, 0.001, 0.002). The predominant pattern was independent A³ (73%, 88/120), with trans-subsegmental arteries most frequently seen (31%, 22/72). Only 24% (19/79) matched the typical B³ bronchial pattern. Arterial common trunks between S³–S^1 + 2 and S³–S^4 + 5 occurred in 55% (66/120) and 30% (36/120), respectively; venous common trunks occurred in 70% (84/120) and 31% (37/120).</p> Conclusions <p>Frequent trans-subsegmental arteries, variable S³a boundaries, and shared vascular trunks complicate surgery, making preoperative 3D reconstruction essential for precise segmentectomy planning.</p>