  Let $G$ be a finite $p$-separable group, for some fixed prime $p$. Let
$\Gamma_p(G)$ be the common divisor graph built on the set of non-central
conjugacy classes of $p$-regular elements of $G$: this is the graph whose
vertices are the conjugacy classes of those non-central elements of $G$ such
that $p$ does not divide their orders, and two distinct vertices are adjacent
if and only if the greatest common divisor of their lengths is strictly greater
than one. The aim of this paper is twofold: to positively answer an open
question concerning the maximum possible distance in $\Gamma_p(G)$ between a
vertex with maximal cardinality and any other vertex, and to study the
$p$-structure of $G$ when $\Gamma_p(G)$ has diameter three.
