To address the significant challenges posed by multiscale heterogeneity in fractured-vuggy carbonate reservoirs, we have developed a comprehensive triple-porosity pressure transient model specifically tailored for slanted wells. A central innovation of this work is the rigorous incorporation of a unified unsteady-state interporosity flow mechanism. This approach accurately describes the dynamic fluid transfer from both spherical matrix blocks and a distinct vug system into the interconnected natural fracture network, thereby avoiding the limitations of simplified pseudo-steady state assumptions. The model also integrates essential near-wellbore effects such as wellbore storage and skin factor. Leveraging source function theory and the principle of superposition, a new semi-analytical solution was derived in the Laplace domain, with the real-time pressure response obtained via the Stehfest numerical inversion algorithm. Based on this solution, characteristic type curves were generated, revealing a key diagnostic feature: two successive troughs on the pressure derivative plot. These troughs represent the sequential fluid supply, first from the more permeable vugs and subsequently from the low-permeability matrix. Extensive sensitivity analyses systematically quantified the influence of well geometry and critical reservoir parameters on pressure behavior. Finally, a field example from a carbonate reservoir validated the model's practical applicability, demonstrating excellent agreement with actual test data. Consequently, this work offers a more robust tool for well test interpretation in these highly complex formations.