“the three sons of Anak.” Anak was one of the Nephilim, the “Fallen Ones,” just as Numbers 13:33 says. This event is recounted in Judges 1:10. Also, the three sons of Anak, Sheshai, Ahiman, and Talmai, are specifically mentioned three times in Scripture (Num. 13:22; Josh. 15:14; Judg. 1:10; see commentary on Judg. 1:10).
[For more on the Nephilim, see commentary on Genesis 6:4.]