Pick a volunteer and give them three dice and a pen and paper.  It might be an idea to give them a calculator as well.  Ask them to roll the three dice once you have turned your back to them.  Then ask them to put the dice in a row and write down the three values shown on the dice to make a three digit number.  They must then append to the three digit number the values on the bottom of the dice, in the same order, to make a six digit number.  Ask them to divide the number by 111, subtract 7 and tell you the result.

You can then tell them what the values of the dice thrown are, without looking, by simply dividing by 9.  For example if they have thrown a 3, 6, and 2.

Three digit number from the values on the top of the dice                362

Append the three numbers on the bottom of the dice                 362415

Divide by 111                                                                                       3265

Subtract 7                                                                                              3258

Then you divide by 9                                                                               362