There is an easy way to know if you have a three-way or an on/off socket.
If your socket has that extra contact, it is a three-way socket, and a three way socket really needs to use a three-way light bulb. Using a non three-way causes flickering and shortens the life of your socket and bulb.
Note: Do not confuse a three-way socket with a three termnal socket. They are not the same thing at all. A three-way socket has three illumination settings: Low, Medium, High(usually 50 watt, 100 watt, 150 watt). A three terminal socket is for a light with more than one bulb i.e. a Gone With the Wind lamp where you turn on the top light, then the bottom light, then both, then off.