  Closed string amplitudes at genus $h\leq 3$ are given by integrals of Siegel
modular functions on a fundamental domain of the Siegel upper half-plane. When
the integrand is of rapid decay near the cusps, the integral can be computed by
the Rankin-Selberg method, which consists of inserting an Eisenstein series
$E_h(s)$ in the integrand, computing the integral by the orbit method, and
finally extracting the residue at a suitable value of $s$. String amplitudes,
however, typically involve integrands with polynomial or even exponential
growth at the cusps, and a renormalization scheme is required to treat infrared
divergences. Generalizing Zagier's extension of the Rankin-Selberg method at
genus one, we develop the Rankin-Selberg method for Siegel modular functions of
degree 2 and 3 with polynomial growth near the cusps. In particular, we show
that the renormalized modular integral of the Siegel-Narain partition function
of an even self-dual lattice of signature $(d,d)$ is proportional to a residue
of the Langlands-Eisenstein series attached to the $h$-th antisymmetric tensor
representation of the T-duality group $O(d,d,Z)$.
