\textbf{151-} Two natural numbers equal to $N = \overline{abc}$, written in the form $\varphi(a \circ bc)$ with a change of base, give the value $N$. What is the largest value of $N$, at least how many units less than a perfect square?
$$1 \ (1) \hspace{2cm} 2 \ (2) \hspace{2cm} 3 \ (3) \hspace{2cm} 4 \ (4)$$
\medskip