Posterior mitral leaflet (PML) hypoplasia is a rare and frequently under-recognized condition in adults, as transthoracic echocardiography (TTE) findings may be subtle. Transoesophageal echocardiography (TOE), particularly with three-dimensional (3D) imaging, is invaluable in confirming leaflet morphology and identifying associated anomalies. We describe three adult patients with PML hypoplasia confirmed by 3D TOE. Common features included rudimentary posterior leaflet, elongated anterior leaflet, and frequent association with congenital anomalies including bicuspid aortic valve and interatrial communications. Mitral regurgitation severity varied from mild to severe. Management was individualized based on symptoms and MR severity, ranging from conservative follow-up to surgical intervention. This series underscores the practical role of 3D TOE when TTE is equivocal: it defines leaflet morphology, distinguishes hypoplastic PML from rheumatic pathology, and detects associated anomalies that influence management. A pragmatic pathway is to suspect PML hypoplasia when MR lacks an obvious mechanism on TTE, proceed to targeted TOE/3D imaging, and tailor treatment to symptoms and MR severity.