Exploring advanced magnetic confinement configurations is crucial for achieving high-gain, steady-state commercial fusion reactors. The negative triangularity (NT) configuration has been listed as one of the potential operation modes for future fusion devices due to its outstanding advantages, including high core confinement performance, good power exhaust capability, and no high heat load on the device wall material caused by edge localized modes. In this paper, the OMFIT was used to study the influence and mechanism of the negative triangularity double null divertor magnetic configuration on plasma confinement performance on the HL-3 Tokamak. Under matched plasma parameters (current, toroidal field, auxiliary heating, and line-averaged density), simulations reveal that the 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 factors of 1.08, 1.35, 1.18, 1.20, 1.02, and 1.49 times those of the PT configuration, 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 the neoclassical transport 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, the turbulent transport is significantly lower in the NT configuration. The analysis of the linear instability of the turbulence mode at ρ=0.32 indicates that under the 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, the 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 in this configuration, which likely explains the reduced turbulent energy transport in the ion channel under negative triangularity.