In this work we study the 3D Navier-Stokes equations, under the action of an external force and with the fractional Laplacian operator $(-\Delta)^{\alpha}$ in the diffusion term, from the point of view of variable Lebesgue spaces. Based on decay estimates of the fractional heat kernel we prove the existence and uniqueness of mild solutions on this functional setting. Thus, in a first theorem we obtain an unique local-in-time solution in the space $L^{p(\cdot)} \left( [0,T], L^{q} (\R^3) \right)$. As a bi-product, in a second theorem we prove the existence of an unique global-in-time solution in the mixed-space $\mathcal{L}^{p(\cdot)}_{ \frac{3}{2\alpha -1} } (\mathbb{R}^3,L^\infty([0,T[))$.