  Chung, Graham and Wilson defined a set of graphs $\mathcal{H}$ to be forcing,
if any sequence of graphs $\{G_n\}_{n \geq 0}$ with $|G_n| = n$ must be
quasirandom, whenever $hom(H, G_n)= (p^{|E(H)|}+o(1))n^{|V(H)|}$ for every $H
\in \mathcal{H}$ and some constant $p \in (0, 1)$. Answering a question of
Horn, attributed to Graham, a forcing set of three graphs is constructed such
that no two of the three graphs are forcing as a pair.
