Let centre of circle be $C(-g, - f)$, then equation of circle passing through origin be
$x^2 + y^2 + 2 , gx + 2fy = 0$
$\therefore$ Distance, $d=|-g-3| = g + 3$
In $\Delta \, ABC, (BC) = AC^2 + BA^2$
$\Rightarrow \, \, g^2 + f^2 = (g + 3)^2 + 2^2$
$\Rightarrow \, g^2 + f^2 = g^2 + 6g + 9 + 4 $
$\Rightarrow \, f^2 = 6g + 13$
Hence, required locus is $y^2 + 6x = 13$