I did a similar trip years ago when I lived in DC, but only in a week and a half (was 2 1/2 weeks total, but we also did San Diego and Las Vegas). With 3 weeks, you will have plenty of time. Here are a few more suggestions based on lessons from that trip and from now living here (LA).

Instead of a day trip to Napa Valley (which we did the first time), do one day out of SF through Sonoma Valley, stay the night somewhere at the northern edge of Sonoma/Napa (i.e. Geyserville), and then come back down Napa on day 2 (or in reverse order).

When you leave Monterey area, drive Big Sur in the afternoon and get into Half Moon Bay in the evening. Have a great dinner, then wake up early the next day for a tour or two of the Hearst Castle (one tour was great - and you have to go all the way back down even if you have a second - but i think we would have enjoyed a second tour quite a bit). From San Simeon, I would make the next stop Paso Robles (drive the afternoon after Hearst). Stick around there for a day at a great B&B and explore the area. Then would drive through Santa Barbara wine country before getting to an overnight in Santa Barbara proper (the 'Sideways' route). Since I love cheezy yet small townish touristy places, I have to say do not skip Solvang.

Maybe a quick visit to/pass-through Ojai on the way from Santa Barbara to LA?

I would keep LA to 4 or 5 days. Plenty of time here. Either take the additional time with day-trips from San Francisco (so many great options), more stops on the way down, or add another stop just south of LA - i.e. Laguna Beach or Newport Beach. Lots of cool festivals (like Pageant of the Masters) going on all summer long.

With 3 weeks, you probably even have time to visit San Diego for two days.