Exploring advanced magnetic confinement configurations is crucial for achieving high-gain, steady-state commercial fusion reactors. The negative triangularity (NT) configuration has been identified as a potential scenario for future fusion devices due to its advantages, including high core confinement performance, effective power exhaust capability, and the mitigation of high heat loads on device wall materials caused by edge localized modes. In this study, OMFIT is used to investigate the influence and mechanism of the negative triangularity double-null divertor magnetic configuration on plasma confinement performance on HL-3 tokamak. Under matched plasma parameters (current, toroidal field, auxiliary heating, and line-averaged density), simulations reveal that NT L-mode confinement reaches the level of positive triangularity (PT) H-mode. Specifically, key figures of merit—β_N, H₉₈, τ_E, W_th, W_th,e, and W_th,i-under NT are 1.08, 1.35, 1.18, 1.20, 1.02, and 1.49 times those of the PT, respectively. Further analysis of plasma thermal transport reveals that the primary reason for the improved confinement performance under the NT configuration is that, within the normalized minor radius range of ρ ≈ 0.2–0.6, the total energy transport (including both electron and ion channels) under the NT configuration is lower than that under the PT configuration. By comparing neoclassical and turbulent transport in total energy transport, it is found that the level of turbulent transport exceeds that of neoclassical transport by 1–2 orders of magnitude. Although the neoclassical energy flux shows little variation between the two configurations, turbulent transport is significantly lower under the NT configuration. Analysis of the linear instability of turbulence modes at ρ = 0.32 indicates that under PT, drift wave turbulence is predominantly driven by the ion temperature gradient (ITG) mode and the electron temperature gradient (ETG) mode. Under the NT configuration, drift wave turbulence instabilities are dominated by ETG and trapped electron mode (TEM), which primarily drive electron thermal transport. Meanwhile, the ITG mode is stabilized under this configuration, which likely explains the reduced turbulent energy transport in the ion channel under negative triangularity.