We develop a comprehensive framework for extracting the pole position and properties of the doubly-charmed tetraquark \( {T}_{\textrm{cc}}^{+}(3875) \) from lattice QCD data using the relativistic three-particle formalism. This approach incorporates the effect of the one-pion exchange diagram in DDπ and DD∗ scattering, making it applicable at energies coinciding with the left-hand cut in the partial-wave projected DD∗ amplitude. We present an example application of this framework to existing lattice QCD data at mπ = 280 MeV. We solve the integral equations describing the DDπ reaction, use LSZ reduction to determine the corresponding DD∗ amplitude, and find the values of the infinite-volume two- and three-body K matrices that lead to agreement with lattice DD∗ phase shifts within their uncertainties. Using these K matrices in the three-particle quantization condition, we describe the finite- volume DD∗ spectrum and find good agreement with the lattice QCD energies. Our results suggest that, at this pion mass, the tetraquark appears as a pair of subthreshold complex poles whose precise location strongly depends on the value of the DDπ three-particle K matrix.
