adjustable range was 5〜24v, It's not the specification.
Product description: "Regulated Adapter LED Power Supply Adjustable Switch Driver With Regulator 3-12V AC/DC Convert Lighting Transformer". In fact output voltage may be adjusted from 3.5 to 12 V, not 3.0 V. At least, it works above 3.5 V and the built-in voltage indication is quite precise (compared to two different digital multimeters).