We introduce a general procedure called `reverse engineering' that can be
used to construct infinite families of smooth 4-manifolds in a given
homeomorphism type. As one of the applications of this technique, we produce an
infinite family of pairwise nondiffeomorphic 4-manifolds homeomorphic to
CP^2#3(-CP^2).Comment: 13 pages, 2 figures. This is the final version published in AGT,
volume 7 (2007), pp. 2103-2116