פּירוש אויף דברים 3:18

ואצו אתכם AND I COMMANDED YOU — He now addresses himself to the sons of Reuben and the sons of Gad.

ואצו אתכם, “I commanded you;” Moses addresses the tribes of Reuven and Gad. The reason he used the pronoun אתכם, “you,” instead of as we might have expected אותם, “them,” is because these two tribes remained part of the Jewish nation made up of twelve tribes owning ancestral land. (Ibn Ezra)

Although Moshe spoke only to the tribes of Reuven and Gad, he said I commanded you because they were a constituent part of Israel. Moshe speaks synecdochically in the opposite sense when he says to the Levites (in the passage beginning with “Take this book of the Torah” [31: 26]) “For I know your rebellious nature” [31: 27]: He was not then speaking only to the Levites, but to all Israel. However, since the Levites were included, he continued to address them.

לפני אחיכם [YE SHALL PASS OVER EQUIPPED] BEFORE YOUR BRETHREN — they used to go in front of the other Israelites into battle because they were mighty men, and the enemy fell before them, as it is said of Gad, (Deuteronomy 33:20) "He tears the arm together with the crown” (cf. Rashi on Numbers 32:17).