Cyclic trinuclear units (CTUs) based on Au(i), Ag(i) and Cu(i) cations, featuring near planar nine-membered coordination rings, represent an important class of metal-organic π-acids/bases with highly adjustable π-acidity/basicity. Their superior π-acidity/basicity coupled with Lewis-acidic and metalmetal bonding sites offers excellent attraction for a wide range of acidic/basic species, and usually followed by noticeable changes of luminescence or charge transfer behaviors. A series of representative cases from the past two decades have been selected herein for such cyclic trinuclear units in both oligomeric and polymeric systems. Their fascinating and profound potential applications related to π-acidity/basicity are highlighted, including molecular absorption and separation, luminescence sensing and detection, organic light-emitting diodes (OLEDs), metal-organic field-effect transistors (MOFETs), molecular wires, and catalysis. The challenges in improving the performance for practical application will also be discussed.