Are 3-watt LEDs 3x brighter than one-watt LEDs?

No, in general, the brightness of an LED source is not cumulative.

Incandescent lighting was simple to measure and compare – a 100-watt lamp was brighter than a 60-watt lamp. Evaluating the brightness of LED sources for various applications requires thinking about differently about light and brightness. High-efficacy LED light sources create more lumens with fewer watts, enabling them to consume less energy than incandescent sources. Also, manufacturers can use different materials within the LED sources themselves, each having its own light-creation efficacy – so two LED sources can consume the same number of watts but differ widely in lumen output. These factors make measuring the brightness of LED sources in watts impossible. The true measure of brightness is delivered light, which measures the actual lumens delivered to the chosen surface, from a desk to a wall to a building façade.