Beijing transportation smart card: can I purchase the card in airport mrt station?

I am sorry that the card is not available at the airport. You can just buy it at customer service centers, metro stations, CITIC Banks, Agricultural Bank of China.
Thus upon landing, you may take Airport Express to Sanyuanqiao or Dongzhimen Station. Then, buy the card at either of the stations. By the way, you need to pay CNY 20 as the deposit, and add some value to it.

Yes. The smart card can be purchased at terminal 3 at the same ticket counter where you purchase the express train tickets. This way you can use your IC Smart Card for your journey on the Airport express train into Beijing and for connecting subway trains or buses. I just did this today so the information is accurate. You can subsequently add money to your smart card at any subway station.