How to wire three gang switch in british colours

SOURCE: 3 LIGHTS TO 3-GANG SWITCH

I draw a little diagram may help you...

SOURCE: how to wire 3 switches in a 3 gang box

WARNING: Do not attempt to do this work unless you completely understand the information provided below. If in doubt call an electrician.
Always make sure the power is disconnected from all possible sources before making any connections. Your description of the problem indicates that there may be multiple power wires coming into the same box. In that case local codes may require that the box be partitioned to prevent exposure to voltages up to 220 volts.
You need to determine where each wire goes to know how to connect them together. You cannot rely on color alone. If the power supply runs through the box to the loads then most (but probably not all) of the white wires will be connected together. If the power runs to the load boxes first then none of the white wires in the switch box should be connected together. They are used to run the switched/dimmed power back to the loads (and should be taped to cover the white in the box). In that case each switch has its own power input and they should not be tied together. A wire (could be black or white with black tape) brings power from the load box, gets connected to the input terminal of the switch and the corresponding black wire carries the switched power back to the load. The fan/light switch will have one power in and a switched power out for the fan and another for the light. The 3-way will have one "power" in (or it could be the final power out to the light) and two "carrier wires that connect to the other 3-way switch. If the power supply goes through the switch box then there will be at least 10 wires in that box. In that case see the attached diagram:

SOURCE: I need to wire a 3 gang switches in a 3 gang box

before u start make sure power is off before working with electrical lines ---start with the 12/2 coming into box black [power] white [ground-neutral] and u will have 3 separate lines for the lights so u will have 4 lines total coming in the box== with the 3 light lines take the white and strip back the plastic and connect [wire nut] all 3 to the 12/2 white line --now take the 12/2 black line and connect it to the first switch gold color screw- bottom screw -now take some wire about 2-3 inch strip both ends and connect from the 12/2 black wire connection on the switch and connect to the gold /bottom of switch 2 than do the same for switch 3 connecting the wire from switch 2 so u will have power feeding all 3 switches-- now connect the black lines for the lights to the other side of each switch and ur done

SOURCE: how to connect three gang

What is 1mm double core?
Following image shows example 120Volt 4 gang switches with outlet.
http://waterheatertimer.org/images/Single-pole-switches-in-4-g.jpg
Each 120Volt electrical box has one Hot wire. This is true for ceiling and wall boxes.
Normally in a gang box, the Hot wire jumpers across all devices in the box.
Some switch boxes do not have Neutral wire. Neutral wires are usually white, and are twisted together and covered with wire nut and shoved to back of box.
Open following links for typical 120V and 240V circuits in US.
http://waterheatertimer.org/B220C.html
http://waterheatertimer.org/See-inside-main-breaker-box.html

Feel free to add a comment and describe wiring, and voltage.
For example if hot wire arrives from one of the lights.
Describe the wires that you have, and what each wire goes to for best possible answer.