Some challenges aren’t directed at beginners but made for rather advanced players. It would be boring if everything were beatable after one week of playing.
However, most quests can be completed with a mix of 3*/4* heroes. Only the last stage of class quests requires a pretty strong team + the last few levels of monthly or seasonal events are close to impossible without the right 4* (or 5*) heroes.
To get most of the 3* heroes, you only need TC13 (+ you occasionally get a 4* out of it). Usually, it doesn’t take long to get either Hawkmoon or–even better-- Belith. Give them some emblems and you already have a pretty good healer.
Even if you don’t manage to get a 3* or better healer early, there are still some ways to win medium hard quests (Gunnar or Kailani are tremendous… Gunnar even brought my 3* + partially leveled 4* team through the last stage of legendary in Guardians of Teltoc…)