Hi as ERIC says this uses an intermediate switch with 4 connections and requires 2 twin and earth cables connected either into one of the 2 way switches or cut into the 3 core/E between the existing 2 way switches.
Don't think this forum has a circuit drawing as not easy to show and many ways to achieve connections none of which are easy DIY!
I would seriously recommend getting a leckie in for this but if you think your ability/knowledge good enough then you need to locate a drawing of what's needed (ie which 2 colours of the 3 core are connected to L1& L2 of switches).
Far to easy to get it wrong for me to try and explain on line,
take care, Sparx