flights from London to Vietnam via Bejing

Hi we are planning to fly from London to Saigon and on the way back stop over in Bejing for 3 days.
we would arrive back in Bejing on 3 May at 11am and flight back to London on 6 May at 5pm. Would this count as 72 hours? would visa be valid till midnight on 6 May?
pls advise before I will by ticket