Flat $SL(3,\mathbb C)$-bundles over a punctured surface are parameterized by conjugacy classes of their holonomy representations. The representations that are completely reducible form an algebraic quotient, and this quotient has the structure of a Poisson variety. When the surface is a once punctured torus or a thrice punctured sphere, we give exact generators and relations defining the parameter space. In the case of a thrice punctured sphere, we also work out the explicit form of the Poisson bracket.