In the third aliyah, the Torah resumes the conversation of G-d and Moshe by repeating elements of it such as Moshe should go speak to Pharaoh, and Moshe answering that he is of "uncircumcised" lips - meaning he felt limited in being able to speak.
G-d says "I am G-d" -- Rashi says that G-d is saying here to Moshe that G-d is competent to send Moshe. Or as it has been said: "When you are doing what G-d wants, you have infinite power".
Moshe is told to go speak to Pharaoh, with Aharon as spokesman, telling Pharaoh that he should let G-d's people go. He is further told that G-d will harden Pharaoh's heart in order to multiply G-d's wonders in Egypt such that Egyptians should know that G-d is the L-rd.
Moshe and Aharon do as G-d tells them, and the aliyah ends by stating that Moshe is 80 years old and Aharon is 83 when they speak to Pharaoh.