Write out every third number starting from 0, but not more than 20.

The first number is 0, the second is 1, the third is 2. So the first third number in the interval from zero to twenty will be number 2. The sequence can be set by the formula:
аn = 3n-1,
where an is a number that satisfies the condition of the problem;
n is the ordinal number of the number.
Then a series of numbers, consisting of every third number, starting from 0, but not more than 20, will look like:
2; five; 8; eleven; 14; 17; twenty.