During a briefing by the White House Coronavirus Task Force, Trump announced that the federal government will fund the missions but all three states will remain in command of the National Guard.
New York and Washington account for about half of confirmed coronavirus cases in the United States.
"We're dealing also with other states but these states have been hit the hardest," Trump said.
Trump said he instructed the Federal Emergency Management Agency to supply four federal medical stations with 1,000 beds for New York, eight federal medical stations with 2,000 beds for California and three federal medical stations as well as three smaller medical stations with 1,000 beds for Washington.
In addition, Washington will receive 369,000 N-95 respirators and 575,000 O-78 surgical masks, and New York will get 186,416 respirators and 444,000 surgical masks.
"This action will give them maximum flexibility to use the National Guard against the virus without having to worry about costs or liability and freeing up state resources to protect the health and safety of the people in their state," he said.
Trump also said that the USNS Mercy Navy hospital ship will be sent to Los Angeles and the USNS Comfort will be sent to the East Coast, likely in the New York harbor.
Inslee sent the 74-page letter of request to the president on Friday to access a slew of federal assistance programs, including crisis counseling and training to lessen the mental impact the pandemic could have on the mental health of his citizens. Other requests made in the letter remain under review, his office said.
In a statement, Inslee said he appreciates the federal approval of the disaster Washington is experiencing, but it isn't enough.
"Today's declaration does not unlock many forms of federal assistance we have requested to help workers and families who are badly hurting," Inslee said. "The state urgently requires additional federal emergency assistance in order to save lives, protect public health and safety and limit further spread of the disease."
Trump's approval of the declaration explains funds have been made available to Washington for crisis counseling and for "certain private nonprofits organizations for emergency protective measures, including direct federal assistance."
In California, Newsom said he asked for the declaration in order to provide "additional assistance, including but not limited to, mass care and emergency assistance, crisis counseling, disaster case management, disaster unemployment assistance, disaster legal service and Disaster Supplemental Nutrition Assistance," according to a statement from his office.
"Unfortunately, California has been disproportionally impacted by COVID-19," Newsom wrote in the letter to Trump. "Besides California being home to nearly 40 million people, which itself poses significant logistical issues few other states face, California partnered with the federal government in several extremely complex and challenging repatriation missions, which strained California's resources and impacted California's healthcare delivery system."