Bearing in mind you'll be looking to sleep someplace each night... you might as well make a couple of those places Portland and Seattle!

How long you spend in each place depends on what interests you and how much time/money you want to spend there.

How's the rest of the trip looking? With three weeks to do this trip you can visit some wondeful places. There are many, many suggestions on this very site (and forum) and we'll be happy to answer any specific questions that you may have.