What is "Eretz Shalishah"?
Radak citing Targum Yonasan: It is land in the south. Yonason similarly translated "mi'Ba'al Shalishah" (Melachim 2 4:42).
The Targum says that he went to the south. We find that Shechem, in Efrayim, was in the middle of Eretz Yisrael from north to south (Makos 10a). Do donkeys go so far?!
Malbim: They do not go so far. It was Hashgachah that he go so far to look for them.
What is "Eretz Sha'alim"?
Radak citing Targum Yonasan: It is a broken land.
Why does it say regarding Har Efrayim, Eretz Shalishah and Eretz Yemini "va'Ya'avor..." (singular), and regarding Eretz Sha'alim it says "va'Ya'avru..." (plural)?
Kli Yakar (Lani'ado) #1: In the three places, they went together as one. In Eretz Sha'alim, they went as two; each searched a different part.
Kli Yakar #2, Malbim: In the three places, they went separately. In Eretz Sha'alim, they went together, as one. Kli Yakar - this is due to danger. We read this like Eretz she'Olim [Sham ha'Oyevim; the enemy ascends there]. Malbim - in the three places, they knew that the donkeys had passed through, so each of them went to a different place, like people looking for something. In Eretz Sha'alim, they did not know that they had been there, so they went together.
Why does it say regarding Eretz Shalishah and Eretz Yemini "v'Lo Matza'u", and regarding Eretz Sha'alim it says "va'Ayin"?
Kli Yakar (Lani'ado): It was not a Chidush that they did not find them in the first place. It was a Chidush not to not find them also in the second place. In Eretz Sha'alim it says "va'Ayin", for they did heard no reports at all that the donkeys had been there. Alternatively, they split up in order to find it sooner - to no avail.
Malbim: In the three places 1 , they knew that the donkeys had passed through. In Eretz Sha'alim, they did not know that they had been there.
He understands that "v'Lo Matza'u" said about Eretz Shalishah applies also to Har Efrayim, unlike Kli Yakar. (PF)