What is the significance of Ashdod, Ashkelon and Ekron?
Rashi: These are governors of Pelishtim.
Malbim: Regarding Ashdod it says Yoshev, for the residents were cut off. Regarding Ashkelon it says "v'Somech Shevet", for the authority was cut off. Also refer to 1:8:3:3.
With Azah in the previous verse, four of the five Sarnei Pelishtim (Yehoshua 13:3) are mentioned. Why is Gas omitted?
Radak (6): It belonged to Malchei Yehudah. Most of them adopted the creed of Yisrael; it says about those who did not "v'Avedu She'eris Pelishtim."
Malbim: It belonged to Yehudah.
What is the meaning of "va'Hashivosi Yadi"?
Rashi: I will strike, and repeat.
Radak: After Azah, Ashdod and Ashkelon are eradicated, I will return to destroy Ekron. Via this, Eretz Pelishtim will perish.
Malbim: This refers to Ekron; it was destroyed twice.
Why does it say "v'Avedu" (plural) "She'eris" (singular)?
Radak: "V'Avedu" refers to the individuals; She'eris refers to the Klal.