Three Week-Long Vacations?
If you want to explore the northwest during a three week long vacation and don't want to be changing motel rooms every night, then one possible solution is to fly into a gateway city in the northwest (not Las Vegas) and take three separate week-long vacations in three separate towns each separated from the other by about a day's drive. Just as one example, you could stay in Seattle WA, Grants Pass OR, and Missoula MT with a final day's drive back to Seattle to catch a return flight home. The advantage is that you can rent entire small homes for a week for not much more than you would pay for a motel room, and you get a kitchen to help lower food expenses, and you don't have to pack every morning and unpack each evening. There are several websites that specialize in hooking up owners of such houses for rent with travelers looking to stay in them. Among these are VRBO, HomeAway, CyberRentals, RedWeek, and others. So, think about that and check it out a little bit. Of course, other city combinations are possible - you really haven't made it clear what it is you want to see and do in the northwest.
AZBuck