In this paper, the numerical work focus on prediction of the thermoacoustic instabilities for a three-dimensional combustor model. A numerical methodology solving the Helmholtz equation with the Flame Describing Function (FDF) in frequency domain is used. In the present numerical work, the in-house numerical code is first examined by the Ri-jke tube benchmark case with the planar flame in a variation of n and τ in one-dimensional n-τ model. Afterwards, the numerical work is extended to simulate the thermoacoustic instabilities of a three-dimensional combustor. The experimental determined FDF is applied in the simulations, which takes into account the non-linear behavior of the flame. In the paper, the eigenfrequencies of the system are presented as numerical results, which are compared with the experimental data.