It appears that your 76.1" width for the Model 3 includes mirrors, while the Prime width you used did not.
Prius Prime length / width / height (excluding mirrors): 182.9 / 69.3 / 57.9 in 
Model 3 width / height (excluding mirrors): 184.8 x 72.8 x 56.8 in 
The Prius is 99% the length, 95% the width, and 102% the height. In terms of frontal area, the Prime has 97% the frontal area of the Model 3.
Overall the Prius has 97% the frontal area of the Model 3.
It appears that the overall drag between the two cars is very similar on paper.
I see your source is Motor Trend, unfortunately their figure differs from the official specs published on tesla.com, which show that the Model 3 width is 76.1 mirrors folded and 82.2 mirrors extended.