If the upstairs switch is down the one in the 3 gang does nothing. Each of the gangs or switches above in fig 2 of which there are three work like this fig 3.

How To Wire Three Switches On One Circuit Electrical Diy
3 gang 2 way switch wiring. Two way light switch mechanism. Find the correct wiring for a 3 way switch in 2 minutes. Way in switch gang in switch and sp dp pole difference in electrical switches. Way in switch gang in switch and sp dp pole difference in electrical switches duration. Three core and earth to one of the 3 gang two way switches 2 core to the 3 gang switch and chandelier wall light single light and another single light. In position 1 when the switch is down or on com and l1 are connected together just like the one way switchin position 2 when the switch is up or off com and l2 are connected together.
Wiring two way light switch diagram uk. 3 gang 2 way light switch wiring diagram uk. In position 2 when the switch is up or off com and l2 are connected together. 3core and earth to single gang 2 way switch. So i am left with the cables in the roof all un connected. In position 1 when the switch is down or on com and l1 are connected together just like the one way switch.
Now in the 3 gang the one that controls a single outdoor light doesnt and the one that controls a landing light on a 2 way with a switch upstairs isnt working as a 2 way should if the upstairs switch is up the downstairs switch in the 3 gang controls the light.