  Let $X$ be a smooth projective curve of genus $g\geq 2$ over the complex
numbers. A holomorphic triple $(E_1,E_2,\phi)$ on $X$ consists of two
holomorphic vector bundles $E_1$ and $E_2$ over $X$ and a holomorphic map
$\phi:E_2 \to E_1$. There is a concept of stability for triples which depends
on a real parameter $\sigma$. In this paper, we determine the Hodge polynomials
of the moduli spaces of $\sigma$-stable triples with $\rk(E_1)=3$,
$\rk(E_2)=1$, using the theory of mixed Hodge structures. This gives in
particular the Poincar\'e polynomials of these moduli spaces. As a byproduct,
we recover the Hodge polynomial of the moduli space of odd degree rank 3 stable
vector bundles.
