  With $G_k(n)$ the number of functions of $n$ boolean variables definable by
$k$-SAT formulae, we prove that $G_3(n)$ is asymptotic to $2^{n+\binom{n}{3}}$.
This is a strong form of the case $k=3$ of a conjecture of Bollob\'as,
Brightwell and Leader stating that for fixed $k$, $\log_2 G_k(n)\sim
\binom{n}{k}$.
