So, I just want to be clear (and selfish). Marjana, Joon, Frida, Kunchen, and Ranvir all useable at 3/70?
I think those heroes can be effectively used at 3/70 that are not hitters or not used because of their dmg. For example healers, they will do their heal % regardless of their attack def and HP stats so the only disadvantage they are having is being easier to kill but their special is at its full potencial. Dmg dealers will deal less dmg because they are not maxed so their attack stat is lower than it could be. Buffers, debuffers and cleansers can also be used at 3/70 like Zim, Zeline, Panther or Frida,bacause their main part is not dealing dmg. So all in all, every 5* is usable at 3/70 that are not hitters or tanks.
Out of the ones you listed: Frida, Kunchen, and Ranvir. Marjana and Joon are snipers first, so they need the levels at 4/* to get their attack near where it should be. While Ranvir and Frida do damage too, their special is more about the buffs or debuffs that go with them.