how do I get from the Ming Tombs to the Summer Palace?

I don't think one-day is enough for you to visit these three places by public transportation. All of them are located far from downtown Beijing and between each other. If possile, you'd better delete Ming Tombs, or ask an agency to arrange a Beijing One-day Tour(https://www.travelchinaguide.com/package/beijing-one-day-tours.htm).

Here are my suggestions on travel by yourselves. In the early morning around 08:00, take bus 919 at Deshengmen station to Badaling Great Wall(https://www.travelchinaguide.com/china_great_wall/scene/beijing/badaling.htm), more convenient than taking a train. At 13:00, take the same bus back to Deshengmen, and then walk to the nearby Jishuitan station of Beijing metro(https://www.travelchinaguide.com/cityguides/beijing/transportation/subway.htm) Line 2. At the next stop of Xizhimen, you should transfer to metro Line 4, which drive you to the Beijing Summer Palace(https://www.travelchinaguide.com/cityguides/beijing/summer.htm) directly.