how do I add athird light switch to an existing two switch l Ask questions and find answers to many subjects relating to electrics and electrical work

[petere]

:(

[ericmark]

Simple answer you don't.
Most light switches do not have a neutral wired to them so extra light must come from existing lamp not switch.
There are diagrams on this site to show how wired.

There is no rule to say they must be wired that way so there will be exceptions but then it's meters out job to find out how wired and not something we can do on a forum