The comprehensive analysis of the phase state, crystal structure and magnetoresistive properties of three-layer films based on alloy Fe$_x$Со$_{1-x}$ ($x \cong$ 0.5) and Cu is done. As shown, a phase state of as-deposited and annealed to 700 K films corresponds to the b.c.c. Fe$_x$Со$_{1-x}$ alloy (single-layer films) or b.c.c. Fe$_x$Со$_{1-x}$ + f.c.c. solid solution of Fe and Co atoms, which isomorphic replace each other in the Cu lattice (three-layer film). Annealing at the temperature of 700 K does not lead to complete mixing of the layers, their original order maintained. For as-deposited three-layer film systems, the isotropic field dependences with maximum values of magnetoresistance (MR) (0.3%) for $d_{\textrm{F}} \cong$ 30 nm (magnetic layers) and $d_{\textrm{N}} \cong$ 5 nm (nonmagnetic layer) are fixed. The process of heat treatment of three-layer samples with $d_{\textrm{F}} \cong$ 20–30 nm, $d_{\textrm{N}} \cong$ 4–15 nm at a temperature of 550 K leads to 4–6 times increase of isotropic MR. Further annealing at a temperature of 700 K causes the appearance of anisotropic magnetoresistance.