Let A be any positive integer,
now, by Euclids Division Lemma,
"a=bq+r"
now, b=3
therefore, 'r' should be 0, 1 or 2 {B'coz 0<r}
now, when r = 0
then,
a=3q+r
a=3q+0
a=3q
========
when r = 1
a=3q+1
when r = 2
a=3q+2
HENCE PROVED