  We compute the disk amplitude of three closed strings in the pure spinor
formalism. Among others, this amplitude probes tree-level gravitational
interactions in the presence of Dp-branes. After disentangling holomorphic and
anti-holomorphic closed string coordinates on the disk by means of introducing
monodromy phases we find a compact expression for the disk amplitude of three
closed strings in terms of open superstring six-point amplitudes. Furthermore,
we provide the low-energy expansion (in the inverse string tension) of our
amplitude and discuss some relevant Dp-brane couplings associated to it.
Finally, we write down an expression for the general structure of the disk
amplitude of any number n_c of closed strings in terms of pure open string
amplitudes involving 2n_c open strings.
