In this paper, we study well-posedness for the following third-order in time equation with delay (0.1) α(M u) (t) + (N u) (t) = βAu(t) + γBu (t) + Gu t + F ut + f (t), t ∈ [0, 2π] where α, β, γ are real numbers, A and B are linear operators defined on a Banach space X with domains D(A) and D(B) such that D(A) ∩ D(B) ⊂ D(M) ∩ D(N); u(t) is the state function taking values in X and ut : (−∞, 0] → X defined as ut(θ) = u(t + θ) for θ < 0 belongs to an appropriate phase space where F and G are bounded linear operators. Using operator-valued Fourier multipliers techniques we provide optimal conditions for well-posedness of equation (0.1) in periodic Lebesgue-Bochner spaces L p (T, X), periodic Besov spaces B s p,q (T, X) and periodic Triebel-Lizorkin spaces F s p,q (T, X). A novel application to an inverse problem is given.