Solution:

Let $x(n)=\delta(n), y(n)=h(n)$

$h(n)=3 \delta(n)$

Condition for stability,

$\sum_{k=-\infty}^{\infty}|h(k)|\lt\infty$

$\begin{gathered}\\ \sum_{k=-\infty}^{\infty}|h(k)|=\sum_{k=0}^{\infty}|3 \delta(k)|=\sum_{k=0}^{\infty} 3 \delta(k)=3 \\\\ \because \delta(k)=0 \text { for } k \neq 0 \text { and } \delta(k)=1 \text { for } k=0 \\\\ \because \sum_{k=-\infty}^{\infty}|h(k)|\lt\infty \text { the given system is stable}\\ \end{gathered}\\ $