Doppler-broadening spectra of positron annihilation radiation for pure metals Ni, Co and Cr and Ni-Co-Cr alloys with three different chemical compositions were measured by 2-detector coincidence technique. And the effect of the 3d electrons on Ni-Co-Cr alloy formation was analyzed. The results show that the 3d electron signal for the metal Ni is relatively higher among the three pure metals. The increase of Ni atom proportion in Ni-Co-Cr alloys leads to an increase in the probability of positron annihilation with free electrons. The experimental ratio curves for three Ni-Co-Cr alloys are greatly similar to the corresponding theoretic ratio curves, which indicate that the bonding nature in Ni-Co-Cr alloys is mainly metallic. This can explain why the Ni-Co-Cr alloys have good metal ductility.