names - Avram-Avraham; Yaakov-Yisroel; Yitzchok-?

Why from the three Avos, Avraham and Yaakov had their name changed, and not Yitzchok?

Answer

Yitzchak's name was bestowed by Hashem (Gen. 17:19).

(Yerushalmi, Berachos 1:6)

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Actually, Rashi (first explanation to Gen. 25:26) says that Yaakov was also named by Hashem. Yefeh Mar'eh simply says that the Yerushalmi evidently agrees with Rashi's second explanation, that he was named by Yitzchak. Tov Ayin, on the other hand, suggests that the difference is that Yitzchak was named before his birth, and Yaakov only afterwards.