  Using the pure spinor formalism, we compute the tree-level correlation
functions for three strings, one closed and two open, in N=1 D=10 superspace.
Expanding the superfields in components, the respective terms of the effective
action for the type I supergravity are obtained. All terms found agree with the
effective action known in the literature. This result gives one more
consistency test for the pure spinor formalism.
