Test of the 12 Marbles

The initial situation is simple. You have 12 marbles of the same size and the same color, perfectly identical to the eye; only one of these differs slightly in weight (the other 11 also have the same, identical weight). 

Using only a balance type "The law is equal for everyone," without other objects nor weights (thus obtaining the information only from the comparison of the weights measured by placing the balls on the scales) and being able to make up to three weighings, you must indicate a method that allows you to identify with certainty the ball differently and if your weight is higher or lower than the others.