Yeah, the + links the words together making it necessary for all 3 to be in a post/thread. If you just put it in with spaces, it will return all posts/threads with those terms individually (many more results). To constrain it more, you can do an exact term by putting quotes around the term using spaces between words. That can artificially restrict the results though.
The 'Search Guidelines' link in my signature runs through the Boolean search options.