  We show that the number of linear spaces on a set of $n$ points and the
number of rank-3 matroids on a ground set of size $n$ are both of the form
$(cn+o(n))^{n^2/6}$, where $c=e^{\sqrt 3/2-3}(1+\sqrt 3)/2$. This is the final
piece of the puzzle for enumerating fixed-rank matroids at this level of
accuracy: the numbers of rank-1 and rank-2 matroids on a ground set of size $n$
have exact representations in terms of well-known combinatorial functions, and
it was recently proved by van der Hofstad, Pendavingh, and van der Pol that for
constant $r\ge 4$ there are $(e^{1-r}n+o(n))^{n^{r-1}/r!}$ rank-$r$ matroids on
a ground set of size $n$. In our proof, we introduce a new approach for
bounding the number of clique decompositions of a complete graph, using
quasirandomness instead of the so-called entropy method that is common in this
area.
