CNN, MS NOW and Politico say their journalists have been denied access to the White House grounds.
It comes after the US president said he was banning the three networks from the area for "reporting fake news".
In a post on Truth Social on Friday night, 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", directly from the US government under Joe Biden, "in order to keep them 'alive'".
He claimed banning the three 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 later 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."
On Saturday morning, MS NOW said its journalists were denied access to the White House grounds.
"The White House belongs to the American people and the decisions made inside are funded by our tax dollars," the network said in a statement.
"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 said its reporter was turned away from the White House grounds on Saturday.
Politico said it was also turned away from the White House shortly after the two other outlets.
Network correspondents are usually there at the weekends when the president is in Washington or at nearby Camp David. Mr Trump is at the presidential retreat in Maryland this weekend.
CNN said it was "fully behind" its White House team and their "fair and accurate reporting".
The channel said it had a right "under the US constitution to do that reporting without hindrance or interference from the government".
Politico said it would "continue to fairly report on this White House and future ones" and defend its "First Amendment rights against any attempt to restrict them".
The National Press Club called on Mr Trump to "reverse his declaration immediately".
Its president, Mark Schoeff Jr, said in a statement: "Banning news organisations from the White House based on the administration's disapproval of their coverage is a clear attempt to control which voices are allowed to witness and report on the exercise of presidential power.
"Every American should understand what is at stake. If implemented, this 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.
The First Amendment of the US Constitution guarantees freedom of the press, meaning they are likely to challenge any ban in court.
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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