One incoming Live wire (Red or Brown) will be connected to one of the COM terminals. A short piece of live wire -a "link" is wired from that first COM to the second COM and another link from the second to the third COM.
L1 or L2 (doesn't matter which) will be used for the live (switched live) running to the light fitting in a one way switching set -up.
So in summary there will be one live wire coming into the switch box and 3 live (switched live) wires leaving it.
There should no neutral wires anywhere near this switch! The switch must always be on the live wire not the neutral!
J
January 2009