Which covenant does "B'ris Melach" refer to?
Rashi and Ramban #1 (citing Midrash): It refers to the covenant that Hashem made with the lower waters at the time of the Creation, that salt would be extracted from it to use for the Korbanos and that some of the water itself would be used for Nisuch ha'Mayim on Succos.
Ibn Ezra: Hashem made a Bris with Yisrael, and commanded them not to bring what is tasteless and not eaten, since it is disgraceful to do so.
Ramban #3: Since this is a covenant in connection with Korbanos, the Pasuk dubs it the head of all covenants, and applies the term "Bris Melach" both to the Matanos Kehunah and to Malchus David. 1
Targum Yonasan: It refers to the covenant that Hashem made with the Kohanim regarding the twenty-four Matanos Kehunah. 2
Moshav Zekenim: Salt comes from water, when the sun shines on it. Water makes vegetation grow, but salt burns and makes infertile. The Bris Melach of Korbanos sustains the world, and cuts off if they transgress it. It is mentioned regarding Malchus Beis David, for salt gives taste to all foods, sustains and cuts off, like Malchus Beis David.
Da'as Zekenim, Hadar Zekenim: Salt lasts. It shows that Hashem has no need for Korbanos; they are only to cleanse Yisrael. Refer to 4:2:151:1.
Which Korbanos does "Kol Korbancha" incorporate?
Rashi: It incorporates Olos Beheimah, Olos ha'Of and the Emurim (the fat pieces) of all other Korbanos.
Why is salt mentioned three times?
Moshav Zekenim citing the Rosh: Salt is put in three places
Which salt is used?
Moshav Zekenim citing R. Yehudah ha'Chasid: It is Sedom salt. Yisrael accepted that if they transgress the Bris, they will be burned like Sedom
Moshav Zekenim: This was said only shortly before Moshe died! We can say that then, Yisrael accepted this Bris (and it is written here).