‘ Also Judah took Gaza, with its border, and Ashkelon with its border, and Ekron with its border.'
These were city states in the coastal plain, from Gaza in the south to Ekron in the north, a distance of thirty to forty miles. No mention is made of Gath or Ashdod, which along with Gaza was where Anakim still survived (Joshua 11:22). These were possibly the cities they did not conquer because they had iron chariots (Judges 1:19). It may even be that the reason that they took these three cities so easily was because the fighting men of the cities had joined those of Ashdod and Gath with a view to defence from an attack by Israel from the highlands, not anticipating an attack from the south. All five cities had been captured by the invading Sea Peoples, the Philistines, and formed the foundation of their state, ruled over by five Tyrants who worked in unison. Note that Judah ‘took' but did not ‘possess'.
Joshua 13:1-3 suggests that the Philistines had already arrived, and extensive excavations at the large mound that is thought to be the site of Ekron have indicated no city after the early bronze age before that built in the early iron age, probably by the Philistines.
But one thing to be considered is that LXX here reads ‘did not take'. This may simply be because it saw it as a contradiction to Judges 1:19, but it may be because it read it in its Hebrew texts. This would in fact find support in the pattern of the narrative. ‘Smote Zephath --- did not take Gaza --- drove out the inhabitants of the hill country --- could not drive out the inhabitants of the valley --- drove out the three sons of Anak --- did not drive out the Jebusites.' In each case a positive followed by a negative. This seems fairly strong support for the negative reading.