Here the key word is combination, i.e. the order of using the machines is not important, it's the choice of the three machines.
One way to calculate:
first machine: 11 choices
second machine : 10 choices
third machine: 9 choices
Total number of ways in order: 11*10*9
But there are 6=3! ways the three machines could be ordered
so the combination is actually 11*10*9/(3*2*1)=165
Another way:
The number of combinations of r choices taken from n objects is
n!/(r!(n-r)!)
=11!/(3!(8!))
=165
as before.
