1 it isn't 3 way switching it is called 2 way switching (there are only 2 ways for the electricity to go)
2 Part P of the building regulations (UK) state that only work certified by competent (i.e qualified electricians) meets the building regulations.
3 Without being conversant to BS7671 (The electrical installation regulations) (which is harmonised to the European regulations) You are risking Fire and or possibly death by indirect contact via improper fusing arrangements/ altering the earth fault loop impedance path.
What you want to do is possible you would need 2 * 2 way 230V switches, 1 * 2 way 230 volt relay (everything rated to the load of your circuit also taking into account the power factor) You do not need to know the state of the relay you could add a 5 V light sensor to the to change a variable 'lightOn' to either 1 or 0
[u]230 Volts can kill and adapting mains voltage circuits with electronics is not for the untrained[/u]
The best idea is make a 5 volt version and then consult an qualified electrician.
*Edit also back switching (mentioned on the site you linked) is unsafe, as switching the neutral leaves main voltage in an unilluminated lamp, further also he makes no note of the conversion method for 2 way switching.
I wouldn't trust that guy to wire a plug.
Nico BS7671 (2008) 17th edition qualified electrician