Reuters also reported that Bloomberg is among the subscribers.
The product, called Truth API, is sold by Trump Media and Technology Group, the company behind Truth Social, and costs financial institutions up to $100,000 a month.
The three named firms declined to comment.
Trump Media officials have put the advantage at 50 milliseconds, which can count for automated systems that scan news and government statements and trade within thousandths of a second.
In April, oil futures fell after Trump announced a ceasefire in the Iran war on Truth Social.
The arrangement has raised concerns about selling faster access to a sitting president's statements, and the deals are likely the first time a business linked to a sitting president has sold such access.
An API, or application programming interface, is a digital bridge that lets one computer system talk to another.
Scrutiny is growing. In July, Senators Elizabeth Warren and Adam Schiff asked the Securities and Exchange Commission to investigate whether the service violates securities laws, calling it a "shocking abuse" of office. Intercept Media and the Freedom of the Press Foundation have sued Trump to stop the White House from posting official announcements exclusively on Truth Social. Trump Media called Truth API an industry-standard product with significant demand and said the reporting relies on misleading anonymous sources. The company told investors in August it had signed more than 10 customer agreements, which Reuters could not confirm. Trump's family trust owns 41% of the company, and its shares have fallen more than 50% this year. Reuters, a Bloomberg competitor, said it has no current plans to subscribe.