As Lifey mentioned, getting from San Diego to Medford in 5 days, via the Grand Canyon, while only driving a maximum of 6-7 hours a day is possible, but it really doesn't leave any time for anything else.

But at the start of your post, you say this will be a 3 week trip? Why are you limiting yourself to just 5 days and what are you planning to do with the other 2+ weeks of this trip? It's entirely possible there is a better way to approach this, where you'd have more time to see parks and natural wonders, without long driving days - but we can't know unless you tell us the rest of the story.