Rashi on Numbers 19:3/רש"י על במדבר י״ט:ג׳

1אלעזר [YE SHALL GIVE IT TO] ELEAZAR — The command concerning it is to be carried out by the Segan (the second to the High Priest) (Sifrei Bamidbar 123:2; Yoma 42b).

2אל מחוץ למחנה [HE SHALL BRING IT FORTH] WITHOUT THE CAMP — Outside the three camps (Yoma 68a; cf. Rashi on Leviticus 4:12).

3ושחט אתה לפניו AND ONE SHALL SLAUGHTER IT IN HIS PRESENCE — A stranger (a non-priest) slaughters it while Eleazar looks on (superintends) (Sifrei Bamidbar 123:2; Yoma 42a).