Police said a woman had been held over suspected fraudulent activity in the process used to vet staff for airside security passes.
The two other women were arrested on 3 August and are currently on bail.
Contractor Sodexo said about 300 employees' airside passes had been suspended "as a precautionary measure".
Scotland Yard said the investigation was not related to terrorism.
A spokesperson said the force was liaising with Sodexo following the arrests but would not reveal any more details about the investigation.
The woman, a 28-year-old from Ealing, was arrested on Thursday morning and is currently in custody at a west London police station.
The Met said her arrest "relates to suspected fraudulent activity in the vetting process for airside security passes at Heathrow Airport".
The two other women from Surrey, aged 24 and 20, were arrested on suspicion of fraud and money laundering after an allegation that money had been fraudulently taken from a bank account.
The 24-year-old is also being investigated as part of the inquiry into the vetting process for security passes, the Met said.
A spokesperson for Sodexo said the company would not comment further "as this is part of an ongoing investigation".
About 70,000 airside staff work at Heathrow.
Hundreds of staff who work at Heathrow Airport have had their airside clearance suspended after three women were arrested on suspicion of fraud.