X-ray magnetic circularly polarized emission (XMCPE) is a novel magneto-optical phenomenon in which characteristic x-rays emitted from a magnetized material are circularly polarized. In this letter, we report a comparative theoretical study of XMCPE spectra of the Kα emission from ferromagnetic Fe, Co, and Ni. Calculated XMCPE spectra have characteristic tail structures on the low-energy side of the Kα emission peaks. Comparison of the spectra shows that Fe, Co, and Ni have broad, intermediate, and narrow tail structures, respectively. Since these tail structures originate from electron excitations in the 3d conduction bands, this difference in tail-structure width can be explained by the difference in the spin-polarized 3d electron states among the three metals.