Can a 3W pot be connected to a 5W LED baton?
First make sure the POT is rated for 240V AC and above. Anyway, after calculating the power dissipation  R x I² = (5/220) * (5/220) *10 000 = 5.165W, It's above the 3W of the pot.

Using this technique to reduce the output of a 240V device is not good because it must be done within the inside circuitry of the appliance, not from outside, on the AC line.
It's even possible that the inner circuit tries to regulate the current of the led to compensate for the resistance you added, increasing the consumption and power dissipation even more.
So even if you can find a 10W ceramic resistor, I would not recommend to use it.

The solution is to modify the electronic inside the appliance itself: Finding how much current the led is taking and then replacing the circuitry by a constant current supply with less current or, if you don't find one, to find out the voltage of the led diode and find an adjustable constant voltage supply.

Buying another appliance may be easier and cheaper.