Bhashya
शितं कर्म किंचित् ब्राह्मणे उत्पन्नं दृश्यते ।त्रयीविहितं कर्मेति प्रसिद्धं लोके ; त्रयीशब्दश्च ऋग्यजुःसामसमाख्या ।मन्त्रेषु कर्माणि कवयो यान्यपश्यन्— इति च आथर्वणे ।तस्माद्युक्तं मन्त्रेषु कर्माप्येकं भवन्तीति ॥
In plain words
No ritual action whatsoever originates independently in a Brahmana text. It is well known in the world that ritual is prescribed by the threefold lore, and the term 'trayi' refers collectively to the Rig, Yajus, and Sama Vedas. As the Atharva Veda declares: 'The actions which the seers beheld in the mantras...' Therefore, it is entirely fitting that ritual actions find their unity within the mantras themselves.
Close to the Sanskrit
Any sharpened action is seen produced in a Brahmana text. 'Action prescribed by the trayi' is well-known in the world; and the word trayi is the designation of Rig, Yajus, and Sama. 'Actions which seers saw in mantras'—and thus in the Atharvana. Therefore it is proper that actions also are one in mantras.