  We explicitly determine the instanton corrections to the prepotential for N=2
supersymmetric SU(3) Yang-Mills theory with massless hypermultiplets in the
weak coupling regions $u\to\infty$ and $v\to\infty$. We construct the
Picard-Fuchs equations for $N_f<6$ and calculate the monodromies using
Picard-Lefschetz theorem for $N_f=2,4$. For all $N_f<6$ the instanton
corrections to the prepotential are determined using the relation between $Tr
<\phi^2>$ and the prepotential.
