This paper deals with the problem of thermoelastic interactions in a functionally graded isotropic unbounded medium due to the presence of periodically varying heat sources in the context of the three-phase-lag thermoelastic models, GN ii (TEWOED) and GN iii (TEWED). The governing equations of three-phase-lag thermoelastic model (3P), generalized thermoelasticity without energy dissipation (GN ii) and with energy dissipation (GN iii) for a functionally graded material (i.e., a material with spatially varying material properties) are established. The governing equations are expressed in a Laplace–Fourier double transform domain and solved in that domain. Then the inversion of the Fourier transform is carried out by using residual calculus, where poles of the integrand are obtained numerically in a complex domain by using Laguerre's method and the inversion of the Laplace transform is done numerically using a method based on Fourier series expansion technique. The numerical estimates of the thermal displacement, temperature and thermal stress are obtained for a hypothetical material. The solution to the analogous problem for homogeneous isotropic material is obtained by taking a suitable non-homogeneity parameter. Finally, the results obtained are presented graphically to show the effect of non-homogeneity on thermal displacement, temperature and thermal stress. A comparison of the results for different theories (three-phase-lag model, GN ii and GN iii) is presented and the effect of non-homogeneity is also shown. In absence of non-homogeneity the results corresponding to the 3P model, GN ii and GN iii model agree with the results of the existing literature.