  We prove an Andr\'e--Oort-type result for a family of hypersurfaces in
$\mathbb{C}^n$ that is both uniform and effective. Let $K_*$ denote the single
exceptional imaginary quadratic field which occurs in the Siegel--Tatuzawa
lower bound for the class number. We prove that, for $m, n \in
\mathbb{Z}_{>0}$, there exists an effective constant $c(m, n)>0$ with the
following property: if pairwise distinct singular moduli $x_1, \ldots, x_n$
with respective discriminants $\Delta_1, \ldots, \Delta_n$ are such that $a_1
x_1^m + \ldots + a_n x_n^m \in \mathbb{Q}$ for some $a_1, \ldots, a_n \in
\mathbb{Q} \setminus \{0\}$ and $\# \{ \Delta_i : \mathbb{Q}(\sqrt{\Delta_i}) =
K_*\} \leq 1$, then $\max_i \lvert \Delta_i \rvert \leq c(m, n)$. In addition,
we prove an unconditional and completely explicit version of this result when
$(m, n) = (1, 3)$ and thereby determine all the triples $(x_1, x_2, x_3)$ of
singular moduli such that $a_1 x_1 + a_2 x_2 + a_3 x_3 \in \mathbb{Q}$ for some
$a_1, a_2, a_3 \in \mathbb{Q} \setminus \{0\}$.
