a person who one distinguishes as someone they are very close to, one they can trust, and one with whom they have fun. It can be considered that a person may have up to three best friends, or may also include a group of three best friends. Therefore, "best" does not just mean only one, and is used as a superlative to express hierarchy. Furthermore, allowing for the song from the comedic film, "The Hangover", to be played and have intrinsic meaning.
We're the three best friends that anybody could have..