assuming you can't repeat a person (duplicates?)
in the 1st chair, there are 5 possible people you can put there
in the 2nd chair, there are 4 possible people you can put there since 1 person already has a chair
in the 3rd chair, there are 3 possible people you can put there since 2 people already have chairs
so there are 5*4*3 or 60 ways to arrange 6 people in 3 seats