A comprehensive study of the three-layer film coextrusion was performed. Plasticized wheat starch (PWS) was chosen as the film central layer, and poly(ester amide) (PEA) was used as the surface outer layers. Single-screw extruders and a standard feedblock attached to a flat coat-hanger die were used to prepare the three-layer films. The layer deformation and interfacial instability phenomena, inherent to multilayer flows, were thoroughly investigated. The effect of process variables, such as viscosity ratio, extrusion rate, layer thickness, and die geometry, were studied. Encapsulation of the central layer by the skin layers readily occurred at the edges of coextruded films. The stability of PEA/PWS/PEA coextrusion flows was closely related to the shear stress at the interface. Increasing global volumetric flow rates and the die gap geometry seemed to promote instabilities. Finally, the existence of instabilities at the interface increased the adhesion strength of multilayered products, due to mechanical interlocking between adjacent layers.