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.

The first year student at the University of Surrey died while returning from a sports trip to Italy.
The other two students have both been treated and are "recovering well", the university said.
The university is working with Public Health England (PHE) to inoculate students living in halls of residence.
All three students were diagnosed with meningitis, but two of them - including the one who died - had the meningitis B infection, prompting the "precautionary vaccinations".
A total of 4,200 full-time students - about one third of the student population - will be offered the jab.
Vice Chancellor Prof Max Lu said the university had been "reassured that the risk to staff, students and visitors is still very low".
Prof Kamila Hawthorne, associate dean for medicine at the university said evidence showed the infection was not "highly contagious, comparatively rare and the risk to the wider community remains very low".
"Only people who have prolonged, close contact with an ill person are at a slightly increased risk of becoming unwell."
Source: NHS Choices

More than 300 bags of rubbish were gathered during the beach cleans at Dun Canna, north of Ullapool.
The tins were from factory ships, known as klondykers, which would anchor in Loch Broom off Ullapool to process mackerel in the 1970s to early 90s.
Almost 50 volunteers were involved in collecting the rubbish.
The effort was part of the Scottish Wildlife Trust's Living Seas project.

Uncapped McCarthy, 26, was called up after Forster, 28, returned to his club for treatment on an arm injury.
The ex-Crystal Palace keeper has played three times for England's Under 21s.
Joe Hart, on loan at Torino from Manchester City, and Burnley's Tom Heaton are the other goalkeepers named in Sam Allardyce's first England squad.
McCarthy joined Saints in the summer as deputy to Forster but has yet to feature in a game.
Goalkeepers: Joe Hart (Manchester City), Tom Heaton (Burnley), Alex McCarthy (Southampton).
Defenders: Gary Cahill (Chelsea), Nathaniel Clyne (Liverpool), Phil Jagielka (Everton), Danny Rose (Tottenham Hotspur), Luke Shaw (Manchester United), Chris Smalling (Manchester United), John Stones (Manchester City), Kyle Walker (Tottenham Hotspur).
Midfielders: Dele Alli (Tottenham Hotspur), Michail Antonio (West Ham United), Eric Dier (Tottenham Hotspur), Danny Drinkwater (Leicester City), Jordan Henderson (Liverpool), Adam Lallana (Liverpool), Wayne Rooney (Manchester United), Raheem Sterling (Manchester City), Theo Walcott (Arsenal).
Strikers: Harry Kane (Tottenham Hotspur), Daniel Sturridge (Liverpool), Jamie Vardy (Leicester City).
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Hundreds of staff who work at Heathrow Airport have had their airside clearance suspended after three women were arrested on suspicion of fraud.
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A student has died from meningitis and another two are recovering, prompting a university to offer undergraduates a vaccination.
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Tin cans from Soviet-era fish factory ships have been found among rubbish cleared from two shorelines in the north west Highlands.
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Injured Southampton goalkeeper Fraser Forster has been replaced by club-mate Alex McCarthy for England's World Cup qualifier in Slovakia on Sunday.