There is no obvious paperwork or other items with a third name, so what they would be able to find depends on what techniques they use and what evidence they look for. For example, the apartment has not been cleaned or wiped down, so forensic evidence such as fingerprints, DNA, and shed hairs from three different people could be found, as well as distinct scents from the three futons that could be used by anyone with scent tracking abilities to identify the individuals if they were encountered. (Unfortunately, it's been long enough since the three of them left the apartment that they can't currently be tracked by scent from the apartment.)

They would be able to collect any of the evidence in the apartment they wanted to keep, such as the junk mail, sheets from the futons for scent or shed hairs, etc. However, anything that they leave in the apartment will be destroyed at the beginning of October, as the rent isn't paid and the landlord cleans out and re-lets the apartment.