The first differences of the sequence are 2, 4, 6.
Presumably the next differences are 8, 10, 12 and the numbers in sequence are 20, 30, and 42; this sequence is
n2 + n for integer n greater than or equal to 0.
When the first differences in a sequence are an arithmetic progression, the sequence is always a 2nd degree polynomial and it is pretty easy to prove that. For an old but thorough reference try to find "Calculus of Finite Differences" by Jordan.