Firstly, we only divide by 3 if we are interested only in the relative seating positions of the guests.
In that case, Homer sitting in seat A, Monty in seat B and Moe in seat C is the same as Homer in B, Monty in C, Moe in A, and also the same as Homer in C, Monty in A, Moe in B.
So 9P3 counts...