I believe it is multiplying by 2 each time.
3 x 2 = 6
6 x 2 = 12
12 x 2 = 24
So if this was true to continue we would just multiply by 2....
24 x 2 = 48
48 x 2 = 96
96 x 2 = 192
192 x 2 = 384
384 x 2 = 768
and continue for as long as you need to go....
Hope this helped!