Which lead came should I use for 3mm glass?

Standard Lead Came has a heart (internal height) of 5mm, most glass for stained glass work is around 3mm thick, this leaves room for cement (waterproofer) which you push into the gaps to seal the panel.  If you are using a thicker glass there is a 7mm heart (high heart lead) which will help accommodate this.  You can choose your lead's width (across the top/bottom) according to your design and the situation.  We recommend a 12mm H section for edging windows which will go into frames (you can trim the edges if needed) and C or H sections for hangings and lightcatchers where the edges are on show.