Can you explain to me how to do these multiplications please?

3•2•1 = 6 - if you break it down it could be done in any way:
(3•2)= 6•1= 6
(2•1)= 2•3= 6
(1•3) = 3•2 = 6

2•3•5 = 30 - same thing as before
(2•3) = 6•5 = 30
(3•5) = 15•2 = 30
(2•5) = 10•3 = 30

4•2•7 = 56 -
(4•2) = 8•7 = 56
(7•4) = 28•2 = 56
(7•2) = 14•4 = 56

3•3•2 = 18 -
(3•3) = 9•2 = 18
(3•2) = 6•3 = 18

Typically you would go from left to right, but any way gets you to the same place. You just multiply the first two numbers then multiply the product of the final value

Hope this helps, have a good day