All three diplomats have raised alarm bells about the release of U.S. security aid to Ukraine being made contingent on Kiev publicly declaring it would carry out politically motivated investigations that Trump, a Republican, had demanded.
Televised public hearings featuring U.S. officials testifying in Congress about alleged wrongdoing by Trump could crowd out other issues like the economy and immigration as voters turn their minds to the November 2020 election.
That might damage Trump, but some of his supporters say the impeachment drive could actually boost his re-election chances by showing him at loggerheads with Washington-based political foes.
Democrats had said they had enough material to move forward with public impeachment hearings, which would be a likely prelude to articles of impeachment - formal charges - against Trump being brought to a vote in the House.
If the House votes to approve the articles of impeachment, the Republican-controlled Senate would then hold a trial on whether to remove Trump from office.
Senate Republicans have so far shown little appetite for removing the president.