what subway line can I get to the Polish Embassy from Beijing Capital Airport Terminal 3?

You can take the airport express line to Dongzhimen Station and then change to line 2 to Jianguomen Station. Get out of the station from Exit B and walk along the Jianwai Avenue to Ritan Road to the embassy.

Or I think you can take the airport shuttle bus line 3 to BJ Railway Station and change a taxi to that embassy. The railway station is about 3km. I suggest you to use this method if you have no idea about the directions of BJ.