What is the significance of the hornet?
Ramban: Hashem filled the air of their land with hornets. 3
According to the Ramban, this hornet (as opposed to the hornet in the time of Moshe) did cross the Yarden, as the Pasuk specifically in Yehoshua (24:11-12). See Ramban.
According to the Gemara in Sotah, 36a, it shot poison into their eyes above, blinding them, and castrated them below (See Torah Temimah).
Why specifically the Chivi, the Kena'ani and the Chiti?
Ramban #1: These three are only samples, 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. 1
Rashi: The Chiti and Kena'ani were in Ever ha'Yarden. Even though the Chivi were west of the Yarden, and the hornet did not cross the Yarden, it shot poison at them from the other side.
How will the hornets drive the nations out of the land?
Rashi: They will strike their eyes, and inject poison and kill them.
Rashi writes that the Chiti and Kena'ani were in Ever (the east side) of ha'Yarden. It says in Yehoshua (9:1) that all six Kena'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.