first:

weigh 6 and 6 ball, leaving 1 ball:
if balance, the left is the answer,
if not, leave the heavier 6 balls for second step;

second:
weigh 2 and 2 balls out of 6 balls, leaving the other 2
if balance, choose the left 2 balls to third step;
if not balance, choose the heavier 2 balls to third step;

third:
weigh the left 2 balls