As @Maryam is specifically using Radiance in OpenStudio, I should mention that there is currently a bug (fixed (which is filed and slated for the next iteration release) correction) in the OpenStudio-Radiance translator that prevents it from working properly on multi-layered constructions. (!)

@Maryam also notes that she has properties for the whole assembly and not the individual elements of the IGU. This is actually perfect for the simple glazing material; just use these properties to make a simple glazing material, apply that single material to a construction, and apply that to your triple pane windows.

Good luck...

-Rob