וֶֽהֱבִיאָ֗הּ אֶל־בְּנֵ֣י אַהֲרֹן֮ הַכֹּהֲנִים֒ וְקָמַ֨ץ מִשָּׁ֜ם מְלֹ֣א קֻמְצ֗וֹ מִסָּלְתָּהּ֙ וּמִשַּׁמְנָ֔הּ עַ֖ל כָּל־לְבֹנָתָ֑הּ וְהִקְטִ֨יר הַכֹּהֵ֜ן אֶת־אַזְכָּרָתָהּ֙ הַמִּזְבֵּ֔חָה אִשֵּׁ֛ה רֵ֥יחַ נִיחֹ֖חַ לַיהוָֽה׃
Und bringe es zu den Söhnen Aarons, den Priestern, und [der Priester] fasse davon eine Hand voll, von dem Kernmehl und von dem Öl sammt allem Weihrauch, und lasse seinen Duftteil in Dampf aufgehen auf dem Altar, als Feueropfer des Wohlgeruchs dem Herrn.
Rashi on Leviticus
הכהנים וקמץ [AND HE SHALL BRING UNTO …] THE PRIESTS: AND HE SHALL TAKE A FISTFUL — from the קמיצה (the taking of the fistful) and onwards is the duty of the priesthood (Menachot 9a).
Rashbam on Leviticus
מלא קומצו, our sages in Yuma 47 explain that what the Torah refers to as קומץ is the amount of flour mixed with oil which a priest can hold when he doubles over the three middle fingers of his hands as if making a fist.
Tur HaArokh
וקמץ, “he shall scoop three fingers full;” this is the first part of the procedure that only the priests were allowed to perform. The procedures mentioned up until this point, i.e. the pouring of oil, the adding of frankincense to the flour before handing the mass to the priest could be performed by the owner/donor, even if he was not a priest. There is only one step in the procedure prior to the scooping up by the priest of three fingers full of the mixture, that also necessitated a priest, and this was the presentation of the mass at the southwestern corner of the altar.