The cases were referred to Metropolitan Police officers earlier this year by the UK's Independent Parliamentary Standards Authority (IPSA) watchdog, which oversees expenses.
The identities of those involved have not been disclosed.
"Two (cases) are being investigated. An assessment of the third referral resulted in a 33-year-old woman, an employee of an MP, receiving a caution in April for fraud by false representation," a Metropolitan police spokesperson said.
The case came to light in an annual report by IPSA compliance officer Peter Davis, where he revealed that he had brought the cases to the attention of the police.
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"Three cases have been forwarded to the Metropolitan Police for assessment; one has been finalised and two remain with the police," he wrote.
The expenses scandal in 2009, uncovered by 'The Telegraph' newspaper, saw seven MPs and peers jailed.
In the wake of the scandal IPSA had banned MPs from being able to make mortgage claims for a second home, among other measures.