The sign: 3 in texting is used to describe that the person texting has mixed feeling about a certain situation. Such signs are called emoticons that help simplify thoughts, actions and feelings. Emoticons are mostly used in social sites.
This guy has the booty under his nose. Pirates want to get their booty(treasure). Picking your nose is called digging for gold.Gold would be considered as booty/treasure.Therefore this is a person happily ignorant to the fact a booger is hanging out from their nose.