Any one have a diagram for 3 way switching. I need to add another switch near the back door for a kitchen light that already is switchable from two other points
If the new switch is in the middle then you need to fit an intermediate switch, this has four terminals. to connect you need to locate the wires that connect to L1+L2, split and wire to new switch so the top connectors go to one switch and the bottom to the other. Will get a bit more complicated if the switch is to be at one end of the room.
mac309 is correct. There will be a pair of red wires already between the two existing switches (called strappers). Cut this (or add new) and connect the intermediate switch.