How do you see as many National Parks as possible in 3 weeks?

Answer 1

If you mean ONLY national PARKS, as opposed to national monuments, then the best way to drive into as many as possible is to start in Seattle, and then drive down the Pacific Coast to see the following:

Olympic, Mount Rainier, Crater Lake, Redwood, Lassen Volcanic, Yosemite, Kings Canyon, Sequoia, Death Valley, Pinnacles, Channel Islands, and Joshua Tree. You could then drive into Arizona to see Grand Canyon and Petrified Forest, then into Utah to see Zion, Bryce Canyon, Capitol Reef, Canyonlands, and Arches. Depending on how unimportant it is for you to actually SEE the parks, you could thus drive by as many as 19 national parks in three weeks.

I emphasize that doing anything remotely like this would mean you would only be driving into these parks, not really EXPERIENCING them. A much BETTER idea would be to decide WHICH parks you want to visit at which time of the year, and then plan to visit no more than eight or so. Experiencing five or six would probably be even better.