The second Posuk in this week's Parsha states "וארא אל אברהם אל יצחק ואל יעקב בקל שקי ושמי ה' לא נודעתי להם," "and I appeared to Avraham, Yitzchak, and Yaakov with the name "Keil Shakkai," but with my name "Hashem" I was not known to them" (שמות ו':ג'). Rashi (שם בד"ה וארא), commenting on the word "וארא," "and I appeared to them," says "אל האבות," "to the Avos," the fathers. What is Rashi adding here? Do we not already know that Avraham, Yitzchak and Yaakov are the fathers? Doesn't the Posuk itself go on to list them by name as the people that Hashem appeared to? Why is Rashi telling us this?
Rabbi Tzvi Dov Kanatopsky answers that the term "קל שקי," "Keil Shakkai," is used in relation to each of the Avos in one specific place. With Avraham, it is used when his name is changed from Avram to Avraham (בראשית י"ז:א'), and with Yaakov it is used when his name is changed from Yaakov to Yisrael (שם ל"ה:י"א). With Yitzchak, whose name is not changed, the term "Keil Shakkai" is hinted to in the Posuk which says "אל תירא כי אתך אנכי," "do not fear because I am with you" (שם כ"ו:כ"ד), because Yitzchak in that Posuk is described as continuing the role of his father Avraham. In each of these cases, Avraham, Yitzchak, and Yaakov are elevated to a new category, it is there that they each become Avos. Hashem identified Himself to each of them for the purpose of establishing an eternal bond with them and to make them into Avos.
So too, Moshe here is also being elevated to a higher category. This category is not that of "Avos" but of "הנביאים אדון," "the master of prophets" or, as the Rambam writes in his thirteen principles of faith, "הוא אב לנביאים," "he is the father of all prophets." For this reason, Hashem used the term "Keil Shakkai" to elevate Avraham, Yitzchak, and Yaakov to the status of Avos, but for Moshe the term "Hashem" is used since he will be elevated to something even more special, to the status of אדון הנביאים, the master of all prophets. Rashi thus stresses that Avraham, Yitzchak and Yaakov were the Avos, because it is specifically when Hashem appeared to them using the name "Keil Shakkai" that they were transformed into the Avos. This highlights the distinction between them and Moshe, who was given a different status, and thus another name of Hashem is used.