  Let $T(n)=\left\{\begin{array}{ll}3n+1&(n\hbox{ odd})\frac n2&(n\hbox{
even})\end{array}\right.$ ($n\in\mathbb Z$). We call "the orbit of the integer
$n$", the set $$ \mathcal O_n:=\{m\in\mathbb Z\;:\;\exists k\ge0,\ m=T^k(n)\}
$$ and we put $c_i(n):=\#\{m\in\mathcal O_n\;:\;m\equiv i\hbox{ mod.}18\}$. Let
$W$ be the set of the integers whose orbit contains $1$ and is, in the
following sense, about well distributed modulo $18$ between the six elements of
the set $I:=\{1,5,7,11,13,17\}$ (the elements of \{1,\dots,18\} that are odd
and not divisible by $3$). More precisely: $$ W:=\Big\{n\in\mathbb
N\;:\;\exists k\ge0,\ T^k(n)=1\hbox{ and }\forall i\in I,\
\frac{c_i(n)}{\sum_{i\in I}c_i(n)}\le\frac16+0.0215\Big\}. $$ We prove that $W$
has density $0$ in $\mathbb N$. Consequently, if the $3x+1$ conjecture is true,
most of the positive integers $n$ satisfy $$ \frac{\max_{i\in
I}c_i(n)}{\sum_{i\in I}c_i(n)}>\frac16+0.0215. $$
