The downloadable zip contains ashader extension allowing for up to 4 detail normal textures masked with a RGBA texture mask to Toolbag 3 shaders.
A texture used to mask associated detail normal maps, red, green, blue and alpha. Independent controls to scale the UV tiling of each detail normal channel.
Download "detail_normal_rgba.zip" from Gumroad
Unzip file to your "C:\Program Files\Marmoset\Toolbag 3\data\shader\mat\custom" folder
Launch Toolbag 3
Select a shader and scroll to the bottom section of the material attributes tab.
Click the triangle on the right and select "Custom"
Click the triangle next to "Shader" and select "detail_normal_rgba"