The paper reports results on the complex study on ferroelectric ceramics that represent solid solutions containing components with a perovskite-type or columbite-type structure. Solid solutions of a three-component (1$-x-y$)${\text{NaNbO}}_3-x$${\text{KNbO}}_3-y$CdNb₂O₆ system are manufactured at $x$ = 0.05–0.20 and $y$ = 0.10. Domain structures in ceramic grains are studied. The consistency between experimental and calculated results is examined for coexisting phases split into non-180${}^{\circ }$ domains (mechanical twins) in the solid solution with $x$ = 0.15. A correlation between the internal structure (crystal, domain, granular, and defect) and fundamental electromechanical and polarization properties is stated for the studied three-component solid solutions.