Is visa required for 3 night stay while waiting to catch the K3 train to Ulaanbaatar

We are a group of 3 adults who will be arriving in Beijing from Melbourne at 22:15 on the 24th of June and will be catching the train to Mongolia at 11:22 on the 27th of June. We will be doing a tour in Mongolia before arriving back in Beijing at around 9:00 on the 14th of July, our flight to Australia leaves later that evening of the 14th. What type do we require or do we qualify for a free transit. Appreciate your help, kind regards John