In a video interview with Axios' Jonathan Swan and Jim VandeHei that aired on Sunday, Trump said he's "not looking to hurt" the U.S. tech giants but is considering action.
"We are looking at [antitrust] very seriously," Trump said. "Look, that doesn't mean we're doing it, but we're certainly looking and I think most people surmise that, I would imagine."
Trump said others have also considered action against tech companies but a "previous administration" stopped that. "They were talking about this years ago. You know they were actually talking about this same subject — monopoly."
Shares of the three tech companies were essentially flat in premarket trading on Monday.
The president has repeatedly attacked Amazon, saying without evidence that package deliveries by the U.S. Postal Service for Amazon were costing the service money.
Trump has also been critical of Amazon CEO Jeff Bezos, who owns The Washington Post.
Mike Allen, co-founder and executive editor of Axios, told CNBC on Monday he thinks Trump was being serious about this inquiry into big tech.
"The president has been talking about this for a long time," Allen said in a "Squawk Box" interview. He also mentioned Trump's "obsession" with Amazon.
Tech companies have been under the microscope in Washington recently on efforts to prevent foreign meddling in U.S. elections.