The Rare tier of Challenge Events is the other obvious 3*-only content.
Otherwise, it depends on the heroes you’re comparing, and whether you can max the 4* heroes, or are limited on Ascension Materials.
In particular, some 3* heroes can be remarkably useful even when you have lots of other heroes leveled. I still use Brienne for my Trials of Fortitude team, for instance, and made good use of her up through 7-8* blue Titans, even after having 4* heroes leveled.