Use the pumping lemma to prove that {www} is not context-free
You could take $w = a^m b a^m$, then you have that $uvxyz = a^m b a^m a^m b a^m a^m b a^m$
$vy = a^l$ with $1\le l\le m$ since there can't be more than one $b$, so you can't pump the $b$.
Since $vy$ can only contain $a^m$ without any $b$'s, it will obstruct the balance between the other $a^m$'s.
We've found a contradiction while applying the pumping lemma so the language is not Context-Free.