CNN, MS NOW and Politico say their journalists have been denied access to the White House grounds.
It comes a day after the US president said he was banning the three networks for "reporting fake news".
In a post on Truth Social on Friday, Donald Trump threatened that more media bans were "to follow".
He claimed MS NOW had recently changed its name from MSNBC "due to lack of viewership and credibility".
And he alleged Politico was the recipient "of an illegal $8m subscription" from the US government under Joe Biden, "in order to keep them 'alive'".
He claimed banning the organisations was due to their "constant 'reporting' fake news".
Mr Trump wrote: "Media outlets shouldn't be able to constantly write or report FICTION and LIES when they're covering the President of the United States, the Trump Administration, or the United States of America.
"Other Fake News Media Outlets to follow."
He told reporters at the White House: "I don't want them in my office. I don't want them here. I would say the ban would go as far as you can."
Asked who might be next, he said The New York Times and The Washington Post were examples of "fake news" organisations, but didn't explicitly state they would be banned.
On Saturday, MS NOW said its journalist Akayla Gardner had been denied access to the White House grounds. She reported that her press badge had been disabled.
"The White House belongs to the American people and the decisions made inside are funded by our tax dollars," the network said.
"MS NOW intends to take any and all steps necessary to defend our First Amendment rights and the essential role of independent journalism in our democracy."
CNN and Politico also said their reporters had been turned away.
Politico editor-in-chief Jonathan Greenberger said in a statement to the newsroom: "We will vigorously defend our First Amendment rights."
The First Amendment to the US Constitution guarantees freedom of the press and freedom of speech among five fundamental rights.
CNN said its White House team produced "fair and accurate reporting" and it would "not be deflected from our duty to hold the government and other public bodies to account".
Betsy Klein, one of its reporters, said her badge had been confiscated by security - who gave her back only the plastic case.
The president of the White House Correspondents' Association, Jacqui Heinrich, also said the moved violated the First Amendment and called for the ban to be immediately reversed.
"The American people, through a free and independent press, must be able to scrutinize those elected to power, regardless of whether government officials view it favorably," she said.
"That's why courts have repeatedly held that once the White House provides access to journalists, it cannot deny that access arbitrarily or based on the content of their reporting.
The National Press Club urged Mr Trump to "reverse his declaration immediately". It said that - if implemented - the ban "would mark an unprecedented assault on press freedom".
Mr Trump has long complained about what he sees as the negative coverage he and his administration receive, coining the phrase "fake news" for those he perceived as unfavourable during his first term in the White House.
He has frequently lashed out at individual reporters in person or over social media, with several recent attacks on CNN's Kaitlan Collins.
In February last year, Associated Press (AP) journalists were banned from the Oval Office, Air Force One and other events in retaliation for the news agency's refusal to follow his lead in changing the Gulf of Mexico's name to the Gulf of America.
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Since returning to office last year, Mr Trump has pursued other legal action against a variety of different outlets, including The New York Times, The Wall Street Journal and the BBC.
AP is fighting its ban in court.
(c) Sky News 2026: CNN, MS NOW and Politico journalists 'denied access to White House grounds' after Trump ban

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