Given r places, we first fill up 3 places by 3 particular things. This can be done in rP3 ways. Now, we have to fill remaining r − 3 places with remaining n − 3 things. This can be done inn − 3Pr − 3 ways. Thus, the required number of permutations will be rP3 × n − 3Pr − 3 ways.
Concept: Factorial N (N!) Permutations and Combinations