<p>Long-lived room-temperature phosphorescent (RTP) films are attractive for information encryption, yet binary host-guest polymer systems often suffer from weak afterglow and limited operational reliability. In this study, we developed a ternary RTP system using poly(vinyl alcohol) (PVA) as the host matrix, coumarin derivatives as the emissive guests, and a third component, boric acids (BA/3-BBA), tannic acid (TA), or metal salts (Ca²⁺, Mn²⁺, Zn²⁺), to regulate triplet-state generation and stabilization. Structural analysis confirmed the guest incorporation and borate network formation. Guest loading decreased the visible transmittance, whereas borate crosslinking partially restored the transparency and improved the thermal integrity (main decomposition near 430 °C). The films exhibit good mechanical performance (for PVA-A: 32.70 MPa tensile strength; 245.60 MPa modulus; and 63.82 MJ·m⁻³ toughness), while crosslinking/coordination increases stiffness (up to 1479.55 MPa modulus for PVA-A-TA). The introduction of the third component enhanced the RTP and photoluminescence quantum yield (PLQY), except for Mn²⁺, which induced quenching. The materials also demonstrated reversible moisture response and good UV stability. Leveraging time-gated afterglow, multilevel information encryption is achieved using dynamic digit displays and an ASCII matrix with decoy and delayed outputs. This ternary strategy enables the practical optimization of PVA-based flexible RTP materials for high-security anti-counterfeiting and information encryption applications.</p>