The construction of metal–organic cages (MOCs) with specific structures and fluorescence sensing properties is of much importance and challenging. Herein, a novel phenanthroline-based metal–organic cage, [Cd3L3·6MeOH·6H2O] (1), was synthesized by metal-directed assembly of the ligand 3,3′-[(1E,1′E)-(1,10-phenanthroline-2,9-diyl)­bis­(ethene-2,1-diyl)]­dibenzoic acid (H2L) and CdI2 using a solvothermal method. According to single-crystal X-ray analysis, cage 1 exhibits a rare trefoil-shaped structure. Meanwhile, the discrete MOCs are further stacked into a 3D porous supramolecular structure through abundant intermolecular C–H···O interactions. Additionally, through exploration of fluorescence sensing on cations, anions, and antibiotics in aqueous solution, the experimental results indicate that cage 1 has excellent fluorescence sensing abilities for Fe3+, Cr2O7 2–, and nitrofuran and nitroimidazole antibiotics. The sensing ability of 1 remains unaltered for five cycles toward all analytes. The above results suggested that cage 1 can be considered a potential multiple sensor for the detection of Fe3+, Cr2O7 2–, and some antibiotics.