here we have a 3 way switching lighting circuit (sometimes called two way switching with intermediate). This allows a single light to be turned on or off from any of the switches. This page shows it wired in the old cable colours if your house has the new colours you want to go here:
The circuit consists of a two way switch at each end (top and bottom switches in Fig 2) and an intermediate switch in the middle. All three switches are connected together by a three core and earth control cable. Notice that the wire connected to the COM terminals is looped straight through the intermediate switch using a cable connector.
All earth wires should connect to the earth terminal in the switch back-boxes and if you are using metal switches there MUST be a loop from this earth terminal to the one on the switch plate (see note A on Fig 2)
Four way switching and more
This circuit can be extended to four, five way switching by adding additional intermediate switches (wired as shown in fig 2) anywhere along the control cable.