"Roll them, and write in sequence the three results, followed by a 3-digit number representing the hidden sides of the rolled dice.
Once you got the 6–digit number, divide it by 111.
THEN CALL ME."
Then he exits and you roll 1,2,4 and write down 124653, divide by 111 and get 1123. The magician upon entering the room gets the result and quips within a second 1,2,4.
How did he do it?