Can be done in two:

Four balls in each pan will show which group has the heavy ball. Split this group between the two pans (two in each). Note which pan (left or right) is low and thus contains the heavy ball. Take one ball from the left pan with your left hand and one from the right with your right hand.

If the scales are even then the heavy ball is in the hand where the ball was taken from the lower pan. Otherwise...

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