The nth term that is killing me

So basically I've got this set of numbers and I've found the pattern but i can't figure out how to into a formula. It's kinda important because it's part of my coursework, which really needs to be posted in the next few days.

So here be the numbers

3,12,30,60,105,168,252

1st set of differences:

9,18,30,45,63,84

2end set of differences:

9,12,15,18,21,24

3erd set of differences:

3,3,3,3,3

I think it's something to do with n^3 but I really can't figure it out.

Help please?