We study the thermoelectric properties of three-dimensional topological insulators in magnetic fields with many holes (or pores) in the bulk. We find that at high density of these holes in the transport direction the thermoelectric figure of merit, ZT, can be large due to the contribution of the topologically protected conducting surfaces and the suppressed phonon thermal conductivity. By applying an external magnetic field a subgap can be induced in the surface states spectrum. We show that the thermoelectric efficiency can be controlled by this tunable subgap leading to the values of ZT much greater than 1. Such high values of ZT for reasonable system parameters and its tunability by magnetic field make this system a strong candidate for applications in heat management of nanodevices, especially at low temperatures. ; Comment: 9 pages, 4 figures, Proceedings of MMM 2011