Both the 3 and the 5 share this sporty look, and the ceramic case.
But I'd say that the 3 is more sober, when the 5 is more dynamic.
Difficult to put in words, indeed.
I'd also say that the 3 has the " magic " of a Tourbilon, which is unexpected on a sports watch, but not unpleasant at all.
I can understand your fascination ofr the 3!
Best,
Nicolas