What is the significance of the hornet?
Rashi and Ramban (citing the Tanchuma): The hornet, which did not cross the Yarden, 1 shot poison into the eyes of the three nations listed in the Pasuk and killed them.
Ramban: Hashem filled the air of their land with hornets. 2
Sotah, 36a #1 (citing Resh Lakish): The hornet did not cross the Yarden with Yisrael. 3 It 'stood' on the bank of the Yarden and and shot poison at the enemy, blinding them above, and castrating them below. 4
Sotah, 36a #2 (citing Rav Papa): There were two lots of hornets; one in the days of Moshe,
See Torah Temimah. Oznayim la'Torah: The eyes and the Ervah
Why specifically the Chivi, the Cana'ani and the Chiti?
Rashi: The Chiti and Cana'ani were in Ever ha'Yarden. Even though the Chivi were situated west of the Yarden, and the hornet did not cross the Yarden, it shot poison at them from the other side. 1
Ramban #1: These three are only examples, whereas in fact, the hornet attacked all seven nations.
Ramban #2: Most men of these three nations opted to remain safely in their fortified cities and not to go and fight Yisrael. So Hashem sent the hornet to destroy them in their homes. 2
Oznayim la'Torah: Because these three nations sinned in the area of adultery more thann the other nations of Cana'an. 3
How will the hornets drive the nations out of the land?
Rashi: They will strike their eyes, and inject poison and kill them.
QUESTIONS ON RASHI
Rashi writes that the Chiti and Cana'ani were in Ever (the east side) of ha'Yarden. It says in Yehoshua (9:1) that all six Cana'ani nations were on the same side (i.e. west) of the Yarden!
Emes l'Yakov: Really, they were west of the Yarden; Sichon and Og conquered them.