We report the observation of quantized conductance in high-mobility three-dimensional Dirac semimetal Cd₃As₂ nanowire and nanoribbon p-n junctions. By employing suspended device geometries with dual local gates, we form tunable p-n junctions and realize ballistic transport across sub-micron channel lengths. In a wide nanoribbon device with a channel width of ∼ 330 nm, conductance plateaus appear at integer multiples of 2e²/h in the n-n regime under high magnetic fields. Numerical simulations suggest that these features represent unresolved spin-split subbands due to the smaller subband spacing in wider channels, and support the interpretation that the observed quantization may originate from surface-state-dominated conduction. In contrast, narrower nanoribbons and nanowires exhibit conductance steps of 1e²/h, demonstrating spin-resolved subbands likely due to enhanced confinement effects. From spin-resolved subband spectroscopy, we extract an effective Landé g-factor of ∼ 43 for the first subband in the bulk gap, establishing these nanostructures as a prospective platform for fault-tolerant quantum electronics.