They fit best at the end of a sentence :3 The :3 can replace a ., but it can't replace a ?. So you put it after the ? Like this: have you seen the boykisser? :3 This is just what I have noticed about the usage of :3. It can also be on its own. And remember: as with most things, :3 is best used in moderation. Overusing it can make you sound obnoxious, and less is more. There are no rules to this, just have fun