This is the Yosemite forum so we've curated topics for the Top Questions about Yosemite. Open the full TA website (not the app) and look to the right.
Enter via the South Entrance to visit Mariposa Grove first.
Then enjoy Yosemite Valley.
Each of those areas (Mariposa Grove, Glacier Point Road, Yosemite Valley, Tioga Road / Tuolumne Meadows) has its own topic in the Top Questions. Also read the Trip Reports, the lodging topic, and any other relevant topic. The maps might help. Three full days would be good.
Weekdays are still busy in September, and weekends are very busy. Plan your visit carefully, get into the parking lot early, and enjoy your time.
For Death Valley NP, use that forum. There are some excellent Top Questions about DVNP.