In chapter 3, what was Elie's first impression of Auschwitz after leaving Birkenau?
I am not precisely clear to which part of the novel you are referring to, but actually the first reference of the camp comes at the very end of Chapter two, when the Jews finally are able to leave their trains:
As the train stopped, this time we saw flames rising form a tall chimney into a black sky... We stared at teh flames in the darkness. A wretched stench floated in the air. Abruptly, our doors opened. Strange-looking creatures, dressed in striped jackets and black pants, jumped into the wagon. Holding flashlights and sticks, they began to strike at us left and right, shouting....
What is important though is when Elie is confronted with the infamous words that adorn the gate to the Auschwitz camp - Arbeit Macht Frei - work makes you free:
First impression: better than Birkenau. Cement buildings with two stories rather than wooden barracks. Little gardens here and there.
So, it is described as being more pleasant. Note too how the commander of the camp, a Pole, speaks the first "human words" to them since their arrival at the camp.
Birkenau was where the new prisoners were received before being sent to Auschwitz. It was at Birkenau that Eliezer and the other Jews were separated from their families and witnessed the burning of people in the crematories. The new prisoners were disinfected and made to change their clothes to prison garb. They were also treated with much cruelty by the older prisoners. Eliezer witnessed his father being physically assaulted, but he could not confront the attacker because of the circumstances.
On arrival at Auschwitz, Eliezer’s first impressions were that the place was better than Birkenau. He based his ideas on how the place looked. Auschwitz featured concrete buildings, which were different from the wooden ones he saw at Birkenau. He also noticed small gardens within the camp’s compound. Generally, the new environment offered him slight relief.