We worked out that you need to make sure that the pile your volunteer picks is always put at the top. This makes their card the 3rd one in the pile.
It does not matter which card they pick or if they tell the truth or a lie because the card will always lands in a certain place when you place them back in the pile.
For example, if they pick the two of clubs. The first time (two) the card will move to 7th place in the pile. The second time (of) the card will move to the 5th place. The last time (clubs) the card will always move to 5th place. So when you spell out MAGIC, the card will always be when you deal the 'C'.
It doesn't work if you increase the number of cards e.g. if you increase it to 4 cards per pile, then the card may not be dealt on the first round as it will be 4th in the pile.