How to find 3 digits and only 3 digits using grep?

Question

I am trying to search all files for the string 111 and only that. Strings as #111 or 13211123 should not be found.

I tried:

grep -rE "1{3}" .

But this is not what I need, I really need to find only 111 nothing in front of it and nothing behind.

Ideas? Thanks!