Potato, sweet potato, and tapioca starches are three of the most commercially important root and tuber starches, yet they exhibit markedly different molecular structures, which directly determine their distinct processing performances such as gelatinization behavior, pasting viscosity, gel texture, and digestibility. Therefore, this study systematically compared the molecular architecture and processing performances of potato starch (PS), sweet potato starch (SPS), and tapioca starch (TS). Chain-length profiling revealed that, compared with PS, both SPS and TS were enriched in short amylopectin chains but depleted in medium- and long-chain fractions. In terms of pasting behaviors, PS gave higher peak, trough, and final viscosities but a lower setback value than SPS and TS. Regarding thermal properties, PS exhibited a higher enthalpy change (13.53 J/g) than SPS and TS, whereas TS showed the highest onset temperature (66.10 °C). For gel texture, PS had the greatest hardness (519.31 g), gumminess (477.98 g), and chewiness (465.49 g). Importantly, regarding digestibility, SPS exhibited the highest level of resistant starch (65.66%) and the lowest level of rapidly digestible starch (10.86%), whereas PS showed the lowest resistant starch (56.36%) and the highest rapidly digestible starch (20.36%). Collectively, these findings are critical for the rational selection of starches for specific food and industrial uses to broaden their application potential.