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This paper uses a parametric numerical study to assess the Lateral-torsional buckling (LTB) performance of several semi-compact beams: S1, S2, and S3. The carrying capacity of these beams, predominantly loaded in bending, is approached by elastic and inelastic buckling analyses. A series of parameters that are believed to influence the resistance to LTB of class 3 beams to (EC3) steel I-beams, namely boundary conditions, flange thickness, and load application level, are investigated. An eigenvalue analysis that predicts the theoretical buckling strength through 3D computational elastic beam models is first conducted using LTBEAM software and ABAQUS. A good agreement in the prediction of Mcr was found. Then, a parametric inelastic buckling analysis is performed using the Riks method implanted in ABAQUS. Results have shown the importance of the lateral restraint conditions and the transverse stiffeners to LTB resistance of compressive flange slenderness following EC3-1-1 for cross sections with a class 3 web and class 1 or 2 flange. In addition, an interaction of local buckling (LB) and LTB in the flanges was observed exclusively for restrained beams. The applied load level strongly affects the beams' elastic and inelastic resistance to LTB.