What does the 3rd symbol on this sword label mean?
my initial guess, the 3rd character is 刻 (u+523b)

http://humanum.arts.cuhk.edu.hk/Lexis/lexi-mf/search.php?word=刻

the seal script in the picture is slightly deviated from standard one.

anyway, i guess your sword is made by 光雲堂, a japanese carving knife manufacturer; maybe you can ask them directly :) 

http://kouundou.com/index.html