Abstract
In this paper we consider a \(3 \times 3\) operator matrix \({{\mathcal{A}}_{\mu }}\) with a spectral parameter \(\mu > 0\) related with the Hamiltonian of a system with nonconserved and no more than three particles on a one-dimensional lattice. Essential and discrete spectra of the operator matrix \({{\mathcal{A}}_{\mu }}\) are described. It is established that the operator matrix \({{\mathcal{A}}_{\mu }}\) has at most four simple eigenvalues outside of the essential spectrum. Spectral estimates for the lower and upper bounds of the operator matrix \({{\mathcal{A}}_{\mu }}\) are obtained using cubic numerical range, Gershgorin enclosures, and classical perturbation theory.