Both Jesus Christ (as the one seated on the cloud) as well as the second angel carried sickles (Revelation 14:14, Revelation 14:17). As Jesus had worked his sickle in Revelation 14:16 (in obedience to the instruction of the first angel in Revelation 14:15), it seems logical that the instruction of the third angel (from the altar) was directed to the second angel of Revelation 14:17.
18 And another angel came out from the altar, the angel who has authority over the fire, and he called with a loud voice to the one who had the sharp sickle, “Put in your sickle and gather the clusters from the vine of the earth, for its grapes are ripe.”