Supersymmetric string configurations with D-branes have been successfully used to provide a microscopic explanation for the entropy of extremal black-holes, the prototipical example being the 3-charge black-hole in type IIB string theory compactified on $S^1\times T^4$. I will discuss how mixed open/closed string amplitudes can be used to study each microstate when the string coupling $g_s$ is switched on and one moves from the free string/D-brane regime to the black hole regime. As an example, I will focus on a particular class of 3-charge vacua and derive from string theory their corresponding geometric description.