Is energy consumption of the 3kw kettle lower than 2kw kettle?
All electric heaters are 100% efficient, at the terminals to its direct environment.
There will however be differences in losses, due to heating things that are not part of the payload.

The 2 kW kettle, with its lower supply current, will cause roughly half the I2R losses in the supply wiring than the 3 kW kettle.
The 3 kW kettle will take less time to boil a certain amount of water. Kettles are not typically insulated, the surface convection losses to the air can be quite high, possibly several hundred watts, so the 3 kW kettle loses less energy to the environment while heating.

That's not the end of the story however, as the two types of kettle will not necessarily be used in the same way, there are psychological factors to consider.

As a 2 kW kettle takes a long time to boil (especially if you used to own a 3 kW version), the user might be more keen to fill it with only the right amount of water. A 3 kW owner might be more careless and overfill it, as they don't suffer so much heating time penalty. This excess water causes two losses. It takes extra energy to heat, which is lost if the kettle cools down between uses. By lengthening the boiling time, it increases the losses to ambient during boiling.