I'd have to agree with others about a visit to Death Valley. It's definitely a place to spend multiple days, even if you live closeby.

But if time isn't on your side, stick to the attractions along your route that you can see from the highway. Artist's Palette, Badwater, Devil's Golf Course, and Titus Canyon, if it's open, could easily fill up your daytime travel.

Titus Canyon is probably better seen when traveling east to west, since the entrance in NV is a fair distance from the exit. The scenery is spectacular, though.

Another really cool place (where there's even a natural spring that produces a rather large waterfall!) is Darwin Falls. That's definitely a 1/2-day event, though, since you have to do a little hiking.

If you're hoping to do spend a day in Vegas, a day in Hoover Dam/Grand Canyon, and a day in DVNP, I'm not sure how you're going to accomplish that, unless you see everything from your car, snap a photo out the window, then drive some more. Where are you starting and stopping your 3 days of travel through AZ/NV/CA?

If you've never been to Vegas, there's enough to see on the strip to fill up an entire weekend. The casinos are pretty amazing.

Hoover Dam is, for the most part, a 3 to 4-hour thing if you leave from Vegas and take the guided tour, and the south rim of the Grand Canyon is another 3.5 hours (one-way) from there.