Let C be a strictly convex domain in a 3-dimensional Riemannian manifold with sectional curvature bounded above by a constant and let Σ be a constant mean curvature surface with free boundary in C. We provide a pinching condition on the length of the traceless second fundamental form on Σ which guarantees that the surface is homeomorphic to either a disk or an annulus. Furthermore, under the same pinching condition, we prove that if C is a geodesic ball of 3-dimensional space forms, then Σ is either a spherical cap or a Delaunay surface.