Most all mulches ( shredded and chipped wood, gritty small chipped pea size stones and tumbled ceramic or glass pieces ) are permeable. They are porous which allows water to pass threw the mass unencumbered. In some cases the mulch can even slow down the velocity of the flow rate due to its mass.

Three inches of uncompacted porous mulch under most circumstances would not adversely change or impact the drainage pattern. It is usually what lies below the loose permeable mulch ( ie, the soil ) that has an impact on drainage flow.

We've used mulch as thick as 6 inches on steep hillsides when doing a sheet mulching project and it did not affect the surface or subsurface drainage flow patterns.