I arrive at Beijing Capital Airport T3 at 07:00, my friend arrives at T2 at 09:00.

Hi Peter, your friend will arrive later than you and you said you will take the airport express train to Dongzhimen after meeting up. So you are advised to first take the free shuttle bus from T3 to T2 and then walk to the boarding position of airport express train at T2 to wait for your friend, which may save much time and is more convenient.

The boarding position for airport express train at T2 is located at the underground two-storied parking lot.