3, 6, 5, 20, 7, 42, 9, 72, 11

Basically there are two series -

Check the alternate number then you will identify there is one series 3, 5, 7, 9, ?

and second series is 6, 20, 42, 74

For 11 approach is 3, 5, 7, 9, 11 -> add 2 to the previous number.

and for 72 approach is 3*2, 5*4, 7*6, 9*8=72