Here, a capsule-shaped cage comprising three monocationic arms was efficiently self-assembled by condensing a triscationic trisaldehyde and a trisamino linkage in water. Multivalence endows the cage with thermodynamic stability in water. Despite its triscationic nature, the cage is able to use its trisimino residue to coordinate a silver cation. As a comparison, other cations lead to cage decomposition or no coordination. The cage and Ag+-coordinated complex were both characterized and confirmed by NMR spectroscopy, mass spectrometry, and theoretical calculations. The metal-ligand complex exhibits a pale-yellow color that can be detectable by the naked eye. The Ag+-coordinated complex undergoes decoordination upon the addition of NaCl, during which the cage containing imine bonds remains intact. Such stability implies that the cage might be potentially employed in silver detection and mining.