All (or the most) discussions on the forum are about 5* heroes. It means that lower level heroes are considered interim in this game. You use them until you got enough 5* heroes.
It is understandable when you make your way through initial game and season 1. You build your base, you build your buildings, collect ascension materials, fight your way through season 1 map etc. Once you are there only 5* become important plus a small number of 4* like Wilbur or Guardian Falcon.
3* of Atlantis or Valhalla are less important because when you get them you already have 4* and 5* that outshine any 3*. Moreover, you hardly can use 3* heroes to go through Atlantis or Valhala maps even at a normal level.
We have nice exceptions like 3* and 4* tournaments and Rare and Epic challenges. But they are not enough to bring attention back to lower heroes.
How about adding more content that will require 3* and 4* heroes.
Introduce separate raid arenas where you can field only 3* or 4* heroes.
Alternatively, give an option to a raid defense to declare that only 3* (4*) can be used to attack it. Or course defense must be built of the same rarity heroes.
Add a modifier to titans: This titan can be attacked by 3 (4) only.
Add the same modifier to Alliance wars. Not only Field aid or Arrrows but hero rarity limit as well.