Two of the men, aged 39 and 44, were arrested at separate addresses in northwest London on Saturday, the Metropolitan Police said.
A 55-year-old man was apprehended at a third address, in west London.
The arrests form part of an investigation being led by the Met's counter terrorism command and are not connected to the arrests of five men - four of whom were also Iranian - also made on Saturday.
The five were arrested on suspicion of preparation of a terrorist act after police uncovered a suspected plot to target a single premises.
Counter terrorism police have been in contact with the site to make them aware and to provide relevant advice and support but it has not been identified for operational reasons, police said.
The five arrested included a 29-year-old man in the Swindon area; a 46-year-old man in west London; a 29-year-old man in the Stockport area; a 40-year-old man in the Rochdale area; and another man in the Manchester area.
Commander Dominic Murphy said it was a "fast-moving investigation".
"The investigation is still in its early stages, and we are exploring various lines of inquiry to establish any potential motivation as well as to identify whether there may be any further risk to the public linked to this matter," he said.
"We understand the public may be concerned and as always, I would ask them to remain vigilant and if they see or hear anything that concerns them, then to contact us."
Police officers are carrying out searches at a number of addresses in the greater Manchester, London and Swindon areas in connection with the investigation.
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