Alright, so there are 4 reindeer to fill 3 different positions, which is a permutation. If you have a calculator, this can be calculated by finding the nPr function and filling in 4P3 (4 pick 3).
If you don't have a calculator, you can use the formula:
n! / ((n-r)!)
where n is the total number in the set and r is the number chosen
solving that formula:
4! / (4-3)! = 4*3*2*1/1 = 24
Either way, you get the answer of 24.