Achieving selective detection of target analytes in aqueous media continues to be an arduous proposition. Herein, we report the conceptualization and synthesis of a novel tailor-fit molecular probe R based on 1,8-naphthalimide which acts as a trifunctional molecular sensor for CN⁻, Fe³⁺ and H₂S. R shows colorimetric and fluorometric “on–off” relay recognition for CN⁻ (red colour and orange emission) and Fe³⁺ (no colour and no emission) in 5% H₂O + DMSO medium which is experimentally ascertained to be a tandem deprotonation–protonation process and is supported by ¹H-NMR titration. Among all RSS (Reactive Sulphur Species), R shows selectivity for H₂S through red colouration. Other coexistent anions, cations and RSS cause no discernible perturbation to the detection process. The detection of H₂S is attributed to a chemodosimetric reduction of the nitro to amino group as evidenced by a potentiometric titration assay. The experimental observations are well supported by DFT theoretical calculation. The K_a for CN⁻/Fe³⁺ are 1.4 × 10⁴ M⁻¹, 6.07 × 10⁴ M⁻¹ respectively and photochemical yield of R + CN⁻ is 0.86. Limit of detections for CN⁻, Fe³⁺ and H₂S are 17.5 nM, 8.69 μM and 8.1 μM respectively. Receptor R is effective for real time applications, bio-compatible and has been successfully employed for confocal fluorescence imaging of RAW264.7 cell and zebrafish.