“the Shihor.” This is some kind of body of water. It could be a river, or Fox calls it a lagoon. Because here it seems to be a southern border of Israel, some scholars think that Shihor is another name for the Wadi El-arish, the “river of Egypt” (Gen. 15:18; Num. 34:5; Josh. 15:4, 47), and this might be the case. Some scholars think it could be an eastern branch of the Nile. The Shihor is mentioned four times in the Bible (Josh 13:3; Isa 23:3; Jer 2:18 (translated “Nile”); 1 Chr 13:5), the precise identification is uncertain.
“which is counted as Canaanite.” The Philistines were not Canaanites, but the area they moved into was broadly considered Canaanite.
“the Gittites.” The inhabitants of Gath were referred to as “Gittites” (cp. Josh. 13:3; 2 Sam. 15:18). The Philistines had five ruling cities, and here in Joshua 13:3 we quite clearly see four of them, Gaze, Ashdod, Ashkelon, and Ekron, but the inhabitants of Gath are referred to as “Gittites,” not Gathites.
“the Avvim.” Inhabitants of the southern Philistine coastal plain, but a group that was not Philistines themselves. They were the early inhabitants of the south coastal region who were displaced from some of their holdings by the people of Caphtor (Deut 2:23), but who continued to live south of the Philistines in the time of the conquest (Josh 13:3) (cp. The Anchor Yale Bible Dictionary. “Avvim”).