Representatives Steve Cohen of Tennessee, Jerrold Nadler of New York and Don Beyer of Virginia on Monday joined a growing list of lawmakers who will not attend Trump's swearing-in at the US Capitol on Friday, AP reported.
The number who initially said they would skip the event has increased after Trump lashed out at House member John Lewis on Saturday for challenging his legitimacy to be the next president.
"This president 'semi-elect' does not deserve to be president of the United States," Cohen said in a statement.
"He has not exhibited the characteristics or the values that we hold dear, that Dr. (Martin Luther) King held dear, that John Lewis holds dear. And when he questioned the integrity of my friend, colleague and civil rights icon John Lewis, that crossed the Rubicon."
Trump and other Republicans have dismissed the boycott and complaints, saying Democrats are sore losers who need to accept the results of the election and move on.
While many Democrats were furious with the outcome of the drawn out 2000 election in which George W Bush defeated Al Gore after recounts and a Supreme Court ruling, they attended Bush's inauguration as the nation's 43rd president.
Obama repeatedly faced questions during the 2008 campaign about the widely debunked claim he was not a US citizen and his birth certificate was a fake.
Trump perpetuated that notion for many years before a brief statement in 2016 that Obama was a citizen.
Republicans attended Obama's two inaugurations.