'三辆' (sān liàng) is a quantified noun phrase meaning 'three vehicles.' It combines the numeral 三 (sān, 'three') with the measure word 辆 (liàng), which is used specifically for wheeled vehicles such as cars, buses, bicycles, motorcycles, and trucks. Unlike general measure words like 个 (gè), 辆 is highly specialized — it cannot be used for people, animals, or non-vehicle objects. This pairing reflects Mandarin’s strict requirement for classifiers when counting nouns.

The phrase appears in contexts involving transportation, logistics, traffic reports, or everyday descriptions of vehicles. Because 辆 is bound to vehicles, saying '三辆' without an explicit noun (e.g., 汽车, 自行车) is common and natural when the type of vehicle is clear from context. For example, in response to 'How many cars arrived?', one might simply say '三辆.' It’s grammatically incomplete on its own but fully acceptable in situational speech.