I'd go to Beijing the day before just to be safe. A three hour delay, which is not uncommon, would set you up poorly for making the connection, plus lately upon landing in PEK I've been sent to bus gates that add at least another 1/2 hour to the experience. Take a late night flight to Beijing the night before, stay in the Langham at the airport and enjoy a nice flight the next day, well rested and relaxed.