Plan to come back. If you just want to walk past the outside of the White House, Capitol, Holocaust Museum, etc., you can do it in a day, but you won’t see much.

The Holocaust Museum itself is worthy of a few hours; the White House and the Capitol also require a few hours each (both the waiting on line and the tour itself).

Then there are the various museums of the Smithsonian and the Library of Congress and just walking around The Mall and monuments. And then there’s the entire city and region beyond the federal core …

Even if you want to do just one or two of the three sites you mentioned, you need to plan ahead. The White House requires you to arrange a tour through your memeber of Congress at least a month in advance. Timed-admission tickets to tour the Capitol are available on the Capitol grounds on a first-come, first-served basis; your member of Congress can probably get you a ticket for a specific date/time so that you can avoid queuing up. Call ahead to get tickets for the Holocaust Musuem.