Why did Moshe insert the word "Gam" three times?
Rashi: To teach us that Hakadosh-Baruch-Hu spent three days at the burning bush convincing Moshe to undertake the Sh'lichus. 1
Ramban: Moshe was simply saying that he had had a speech defect ever since his youth, and that same defect was still present. 2
Why did Hashem not cure Moshe's defect?
Ramban #1: Because he did not ask Him to. 1
Ramban #2 (citing the Midrash): Hashem specifically wanted Moshe's speech defect to remain, to add to the miracle. 2
Ramban #3: It was the result of Hashem's anger 3 - a punishment for refusing to accept the Sh'lichus.
Moshe, for his part, did not ask Hashem to cure him, because he did not want to go (Refer to 4:10:3:1). So he thought that, since it is not dignified to appear before Pharaoh in the Name of Hashem in that state, Hashem would surely pick someone else to go (Ramban).
That a person who could not speak properly was able to converse with Pharaoh and issue him with instructions flawlessly.
See Pasuk 14.
Since he fled the country in his youth and now (upon his return) he was eighty (Rashbam).
What was Moshe's basic reason for refusing to accept the Shl'ichus?
Rashi: Because he did not want to accept a position that placed him higher than his brother, Aharon. 1
Ramban (on Pasuk 13): It was due to his extreme humility; 2 He simply could not conceive aggrandizing himself, to speak to the king and tell him that Hashem had sent him, and to take Yisrael out of Egypt and to be their king.
Moshav Zekenim (3:12): He feared lest Pharaoh want to kill him. 3
Who was (three years) his senior and who was already a Navi (as the Pasuk in Shmuel 1 (2:27) indicates (Rashi).
Rashi writes that "Gam" is written three times to teach three days. We hold that an inclusion after an inclusion comes to exclude!
Moshav Zekenim: An inclusion after an inclusion comes to exclude only when they include the same matter. Here, they include different days.
What is "Lo Ish Devarim"?
Hadar Zekenim: Sheluchim of the nations know all 70 languages. I do not know even mine!