Excuse me for being silly, but who said standard phone headsets are 3 conductors? It's impossible to make a stereo headset with a microphone with just 3 conductors. Well, okay, not theoretically impossible. But very hard to manage nevertheless.
Secondly, there are two different kinds of Nokia headsets. The Nokia 5200/5300 headset has some sort of electronics built into it for the radio function on the phone. Some other Nokia phones use standard stereo headsets (4 conductor, of course).
I will have to say that all the standard stereo headphone are using 3 pin connectors. One for left, one for right and one is ground.
Secondly, there is no special electronic inside the headset for the radio function. Nokia design the phone in such a way that only when you connect a headset to the phone can you hear the radio. The reason behind this is that you need an FM antenna for the radio to work. The headset is the antenna. It is just using the impedence sensing trick to found out if you have connected the headset or not.