Good question Dues. I guess only in white Buca? I just saw this on Wikipedia:

Three beans in sambuca means
A serving of sambuca can be a shot with seven coffee beans, representing the seven hills of Rome. Likewise, a shot with one coffee bean, called con la mosca, which means "with the fly", is as common. The traditional serving is with three coffee beans, each representing health, happiness and prosperity.


I always thought it was the Father, son and the holy spirit!