Adding Commas or Separator to Compiled Values

So I'm trying to fetch the form "lesson material"'s module name ("module_text"), subject name ("subject_text") and category("category"):

FetchMaterial = Lesson_Materials[ID == input.Lesson_Materials];

Then I'm trying to insert those 3 things (module name, subject name, and category) into a multi-line text field:

input.Multi_Line = FetchMaterial.Module_Text + FetchMaterial.Subject_Text + FetchMaterial.Category;

This works fine, but when there is no spacing or separator between the 3 things. So I end up with something like this:

ModuleSubjectCategory

I want it to be like this:

Module, Subject, Category

How can I do this?

Thanks