Grammar for language L on {a, b} where L = {w|na(w)mod 3 = 0}

Question

I am able to form the regular expression but I am not confident with the grammar. I have tried the following:

S-->aaaS|bS|b|lambda

Regular expression is given by:

b* +(ab*ab*ab*)*

Somebody please guide me.

Zulfi.