feat(strings): add strings3

info.toml

@@ -443,6 +443,17 @@ Yes, it would be really easy to fix this by just changing the value bound to `wo
 string slice instead of a `String`, wouldn't it?? There is a way to add one character to line
 9, though, that will coerce the `String` into a string slice."""
 
+[[exercises]]
+name = "strings3"
+path = "exercises/strings/strings3.rs"
+mode = "test"
+hint = """
+There's tons of useful standard library functions for strings. Let's try and use some of
+them!
+
+For the compose_me method: You can either use the `format!` macro, or convert the string
+slice into an owned string, which you can then freely extend."""
+
 # MODULES
 
 [[exercises]]