BIG ISSUE! I need to enter PRC 3 times in 15 days, anyone can give me some advice?

I will entry in China 3 times in 15 days!! First I arrived at International Airport Beijing at 30th August.08:30 am and I depart by the same airport next day at 9:30 am to Mongolia. I have already flight ticket and seat confirmed by Mongolian Airlines. Should I apply for the "24h Direct Transit" or "72 hours visa exemption"? On 4th September I will arrived at International Airport Beijing at 20:10 and I will pick up the train Beijing- Dandong- Pyongyang (North Korea) - K27- on 6th to Dandgong and 7th at 8:30 am to Pyongyang (less than 72 hours). in this case what should I apply for? in my 3rd entrance at 10th, 16:30 pm, in Dandong I think I must have a Tourist L type for stay in Beijing 4 days.....Anyone can help me? Thanks in advance! :)