Context-free grammar for $a^ib^ic^id^{3i}$
You can use the pumping lemma to show that this language is not context-free.
Suppose it is context-free and let $n$ be the constant of the pumping lemma. Consider the word $u = a^nb^nc^nd^{3n}$. $u$ can be written $u = vwxyz$ with:

$|wxy| \leq n$
$|wy| &gt; 0$
$\forall k \in \mathbb{N}, vw^kxy^kz\in L$.

Now using the first condition, $wxy$ can be written with at most two letters from the initial four. You can take it from here to find a contradiction.