second postscript
Used in letter writing to add a note after a first postscript (二伸). The number 三 indicates it is the third addition overall, following the main text and the first postscript.
At the end of the letter, I wrote 'second postscript' and added one more matter.
From 三 (three) and 伸 (extend, add), referring to the third addition in a letter after the main text and the first postscript.