Cohen, Trump's fiercely loyal and pugnacious lawyer, was in court to ask a judge to limit the ability of federal prosecutors to review documents seized from him last week through a warrant as part of a criminal investigation.
The US Attorney's Office has proposed to use a so-called "taint team" to conduct the initial review - lawyers from the office who are not involved in the Cohen investigation - to determine what, if any material, should be shielded from the investigators because it is protected by attorney-client or another privilege.
The third is said to be Sean Hannity.
Attorneys for Cohen had already applied to Judge Kimba Wood at Manhattan federal court for a restraining order that would stop authorities reviewing the records seized from Cohen until Cohen's team has reviewed the materials themselves.
As part of the judge's effort to resolve such requests, she asked Cohen's lawyers to tell her Monday how many clients, and documents, might be involved in the material seized by the Federal Bureau of Investigation.
The letter says that from 1996 to 2006, Cohen had hundreds of clients, adding that he did not know if any material from those old clients were in the seized files.
Last week's raids came after a "months-long" investigation related largely to Mr Cohen's business dealings, rather than his work as a lawyer, prosecutors have said. They, in a filing on Sunday night, made clear that they believe Cohen and Trump's lawyers should receive copies of the seized materials to make their own assessment of which documents are privileged.
Michael Cohen outside court in NY on April 16.
The other two clients were President Trump and Elliott Broidy, both of whom involved hush money deals over illicit affairs. The attorneys said in the filing that Cohen represented three clients in the past year: Trump; Elliot Broidy, the Republican fundraiser for whom Cohen helped facilitate a $1.6 million payment to a Playboy playmate; and a third person. He said Cohen reached out to him after being contacted by Keith Davidson, an attorney representing the woman, and said he retained Trump's lawyer "after he informed me about his prior relationship with Mr. Davidson".
Cohen's lawyers argue that if the court appoints a special master to oversee the issue, "that legal client would allow their name to be disclosed to the special master".
In making their requests, Trump and Cohen contend that the Justice Department policy is unfair and infringes on constitutional protections.