Can I replace a Maximum Bulb Wattage (Watts) 25 halogen light bulb with 5w LED light bulb? Is it safe?
When choosing lamps, as long as the actual wattage is below the rating of the fixture, you're safe.  It doesn't matter that the fixture generates a lot of light with the watts it consumes.  

The equivalent wattage they talk about, like where they say this 3W LED has light output equivalent to a 15W halogen, doesn't matter when selecting lamps for a fixture.  The 3W actual wattage determines how much heat it may generate, not the 15W equivalent light output.  

In fact it's likely that the LED is more efficient, with less of the electricity consumed turning into heat and more into light, but you can't count on that.  Just go by the actual wattage.