Rashi

יהודה אתה יודוך אחיך THOU JUDAH, THY BRETHREN SHALL PRAISE THEE — Because he rebuked the first three tribes in harsh terms Judah began to move away that he (Jacob) should not censure him for the incident with Tamar; Jacob therefore recalled him with words that would soothe him: Judah, you are not like these! — you your brethren may indeed praise (Genesis Rabbah 99:5). ידך בערף איביך THY HAND IS ON THE NECK OF THINE ENEMIES — a prophecy fulfilled in the days of David, a descendant of Judah, who said (2 Samuel 22:41) “Thou hast also made mine enemies turn their back unto me” (Genesis Rabbah 99:8). (The Hebrew may signify, “Thou hast given me my enemies’ necks”) בני אביך THY FATHER’S CHILDREN — Because they were born of different wives he did not say “thy mother’s children” as Isaac said (Genesis Rabbah 99:8). (He meant: all thy brothers will bow down to thee. Had he said “thy mother’s children”, he would have included only Judah’s brothers who were born of the same mother as himself.)