1 is the
language code for a Wikisource. Leave "1" blank, or assign to anything other than a language prefix, in order to link to the multilingual Wikisource (
wikisource.org).
2 is the page to which you're linking. Optional.
3 is an optional parameter: the text that may appear instead of 2. Optional.
If neither 2 or 3 are specified, the pagename will be used.