I’d suggest going to Scotty’s the second day and returning to Furnace Creek for the night. If you went to the Castle on the third day, you would be driving 50 miles north of Furnace Creek, in the opposite direction from Las Vegas. So in addition to the time you spend at the Castle, you’ll also add 100 miles or so over the distance from Furnace Creek.
I’m not sure exactly what order you’ve decided on for your sightseeing, but if you have a good part of Day 2 to spend on the Castle trip, you can also include Ubehebe Crater and the west end of Titus Canyon. Ubehebe Crater is a volcanic site about 10 miles from the Castle, on a marked road north of the (unstaffed) Grapevine entrance station. It was created by a volcanic implosion and you’ll see volcanic pumice soil and rocks all around. It’s possible to hike into the crater and back, or around the rim. Titus Canyon is a drive-through where 4wd and/or high clearance is often needed due to road and weather conditions, but you can drive to the west end in any vehicle, park, and walk wherever you like. The canyon walls are narrow and sheer, and show the effects of flashfloods in the desert.
Another side trip, a bit farther, is Rhyolite ghost town. It’s about 60 miles from Scotty’s Castle and 40 miles from Furnace Creek. It is one of the more extensive remaining ghost towns in the DV area, with substantial stone and concrete ruins and foundations. If you don’t want to go that far, you could go to the Sand Dunes that same day; they are only about 7 miles west of the junction of Hwy 190 and Castle Road.
If you have all of Day 2 for the Castle trip, you could include most or all of these other places. If you wait until Day 3, as I said above, you’ll drive farther and have less time for sightseeing, and the ride to Las Vegas afterward will be much longer.
If you do decide to go to the Castle on the third rather than second day, you can go to Las Vegas on Hwy 95 via Beatty and see Rhyolite, which is 5 miles west of Beatty. Other than that, there’s no scenic advantage to Hwy 95. It isn’t ugly, but it’s monotonous compared to Death Valley. This is another good reason to visit the Castle on the second day. You can savor the more dramatic DV scenery on the Castle round trip, have a relaxing last evening at Furnace Creek, and still have a few hours in the morning if you wish, to catch anything you missed or want to revisit around central Death Valley. Las Vegas is about 2 hours from Furnace Creek, so yo could do some final exploring and get to town at a reasonable time and not be fatigued.