Three of Five

From Memory Alpha, the free Star Trek reference

You might also be looking for the drone known as Third of Five.

Three of Five, Tertiary Adjunct of Unimatrix 01 was a Borg tactical drone, outfitted with a tritanium infrastructure, who formerly served near or for the Borg Queen, during the 2350s.

Prior to his assimilation, Three of Five was a Ktarian male. As Borg drone, he would be identified and tagged by Magnus and Erin Hansen, who deemed him as an extremely important drone. (VOY: "Dark Frontier")