In how many ways can you sit 8 people on a bench if 3 of them must sit together?

for this Q i can understand that we can consider those three ppl as one person then the answer is 6!*3!=4320

however, most time i will be confused when we need to divide total ways by the number of position for instance 8!/3!...i learned that when we have indistinguishable objects in the set wee need to divide by the # of those objects.....plz explain when we use multiplication, when we have to use division..