Hello Syed
This is directly by Euclid's Division Lemma,
If a and b are two positive integers, then
a = bq + r,
where 0 <= r < b
So,
for b = 3
a = 3q + r
where 0 <= r < 3
So, r can be 0, 1, 2
So, a = 3q, 3q + 1, 3q + 2
Where a is any positive integer