Steel portal frames, used in factories, workshops, shopping complexes, and warehouses, provide large clear spans. Eurocode 3, a semi-probabilistic design code based on limit state design and adopted in Nigeria, does not fully address uncertainties in load and resistance variables, affecting frame performance in service. This study evaluates the reliability of three-hinged steel portal frames by analyzing three primary failure modes: flexural instability of frame stanchions (failure mode 1), flexural instability of frame rafters (failure mode 2), and lateral torsional instability of stanchions and rafters (failure mode 3). Limit state functions for these failure modes were derived from Eurocode 3 specifications and structural analysis load effects. Stochastic models of uncertain parameters are obtained from the literature. The First-Order Reliability Method (FORM), using a MATLAB program, evaluates the limit state functions and determines failure probability. Results show that for failure mode 1, Eurocode's target reliability of 3.8 is met if Xc is at least 0.85 for stanchions and 0.6 for rafters. At flexural buckling (XLT = 1.0), lateral-torsional stability yields a safety index of 5.8. For failure modes 1 and 2, the safety index decreases with a higher dead load to variable load ratio. For all failure modes, the safety index increases with higher steel grades and coefficient of variation (CoV). To ensure safety, the study recommends fully accounting for uncertainties in design to prevent up to a 60% compromise in portal frame safety.