The names suggest that the first takes whatever text is selected, transforms it, and replaces the selected text with the transformed text. The second suggests that it takes whatever text is selected, transforms it, and copies the transformed text. The third suggests that it takes the clipboard and replaces it with transformed text.
However, they all seem to act on the clipboard rather than the selected text, and they all seem to do exactly the same thing: set the clipboard to the transformed text. Are there any differences?
Update: I found that if I change the clipboardContentString variable in the template to selected_text, they do replace the selected text with transformed text, but they all seem to work exactly the same: run the script. Maybe the template text for each is supposed to be different?