In this effect which requires no technique, the magician asks a spectator to choose a symbol among nine others and to hide it.
Then, the spectator turns over the cards revealing the numbers and makes three additions, being free from start to finish in positioning the order of the cards, and therefore in choosing the three numbers.
As soon as the magician knows the result of the addition, he immediately reveals the chosen symbol.
Of course, during ALL these operations, the magician turns his back on the spectator.
Here's a trick you could even do over the phone!