fourth
The lady I got them from says they're called shamrocks, and apparently they're one of the symbols of an Irish holiday called St. Patrick's Day (she said everyone's Irish on St. Patrick's Day).
The lady also had T-shirts with funny things on them like 'Rub me for Luck' and 'Have a Fookin' Brilliant Day!' (though most of them were about beer), but none of them seemed right for Yuu.