We are only visiting 3. Natural history. American history. And air and space. Should this be done on two days or can we do it in one? We are looking to go in April. Week leading to Easter. Wednesday thru Sunday.
Visiting the three Smithsonian museums you mentioned — the National Museum of Natural History, the National Museum of American History, and the National Air and Space Museum — can be done in one day, but it will be very rushed, especially if you want to see highlights rather than just walk through quickly. These museums are large and very popular, particularly the Air and Space Museum, which tends to get crowded, especially in April during the week leading up to Easter.
Here are some considerations:
In summary, while it is technically possible to see all three in one long day, splitting them over two days will make for a much more enjoyable and less stressful experience.