Notepad ShaderStudio is good also I don't know any compatible program. But try to search in shader thrad. There are some help posts, describing needed changes in shader code to put it into 3DGS from RenderMonkey or alike.
I liked working with RenderMonkey... of course like any other shader program, it must be converted, but I found that I liked using the program itself and the example files that come with it.
