In this paper, we obtain an analogue of Toponogov theorem in dimension 3 for compact manifolds $$M^3$$ M3 with nonnegative Ricci curvature and strictly convex boundary $$\partial M$$ ∂M . Here we obtain a sharp upper bound for the length $$L(\partial \Sigma )$$ L(∂Σ) of the boundary $$\partial \Sigma $$ ∂Σ of a free boundary minimal surface $$\Sigma ^2$$ Σ2 in $$M^3$$ M3 in terms of the genus of $$\Sigma $$ Σ and the number of connected components of $$\partial \Sigma $$ ∂Σ , assuming $$\Sigma $$ Σ has index one. After, under a natural hypothesis on the geometry of M along $$\partial M$$ ∂M , we prove that if $$L(\partial \Sigma )$$ L(∂Σ) saturates the respective upper bound, then $$M^3$$ M3 is isometric to the Euclidean 3-ball and $$\Sigma ^2$$ Σ2 is isometric to the Euclidean disk. In particular, we get a sharp upper bound for the area of $$\Sigma $$ Σ , when $$M^3$$ M3 is a strictly convex body in $$\mathbb {R}^3$$ R3 , which is saturated only on the Euclidean 3-balls (by the Euclidean disks). We also consider similar results for free boundary stable CMC surfaces.