Why should Yakov accompany Leah after three sons?
Rashi: She knew that Yakov will have 12 sons from four wives, so she gave birth to her proper share.
Riva citing R"M mi'Kutzi: I can handle only two. Now he will need to help me. 1
Tosfos ha'Shalem (3, citing R. Efrayim): I can handle one, and my Shifchah can handle one. Yakov will need to engage with the third. 2
Tosfos ha'Shalem (5): Yakov must teach the oldest Torah and Yir'as Shamayim, and I will deal with the younger.
Ohr ha'Chayim #1: One could attribute the first two sons to removing hatred, and causing Yakov to love her. Her third son must be because she is his Zivug; Hashem caused that he marry her.
Ohr ha'Chayim #2: He will accompany her after leaving this world, for she is his Zivug. Three sons make a Chazakah! Also, she knew through Ru'ach ha'Kodesh that Yakov will have 12 sons through four wives (so she has a proper share; surely she is a Zivug of him).
Malbim: The hatred has abated, so this time he should accompany me.
Regarding the other sons, the mother gave the name. Why does it say here "He called him"?
Rashi: Hashem called him Levi, because he accompanied him with 24 gifts of Kehunah.
Rashbam: Yakov called him Levi. 1
Tosfos ha'Shalem (9), Ha'Emek Davar: He saw with Ru'ach ha'Kodesh that it would be written about Levi "v'Nilvu Alecha vi'Sharsucha." Malbim
Why does it say "Al Ken" regarding calling the names of Levi, Yehudah and Dan?
Ha'Emek Davar: For these, the name reflects the destiny of the [Shevet of the] child. 3 E.g. Levi is based on Leviyah; he will be more connected to Hashem than the other Shevatim.
QUESTIONS ON RASHI
Rashi writes that Levi did not have many people, for the Aron was Mechaleh. Levi was the smallest Shevet counted at the beginning of Bamidbar, before they carried it!
Riva, citing R"M of Kutzi: The Shevatim increased according to how much the Mitzriyim afflicted them. Levi was not enslaved.
Divrei David: Because they carry the Aron, they needed great Kedushah. The Aron was Mechaleh, i.e. caused that there be few Leviyim. 1
Only Bnei Kehas carried the Aron. Kehas was bigger than the other families of Levi (Gershon and Merari