I have stopped competing in the Rare level of the events. I do one run of level 1 just to get the free token, but the completion rewards aren’t worth the time and world energy, which I can better use to compete in Epic or Legendary.
Honestly, unless you are really willing to invest in getting top-10, there’s not a lot of rewards in Rare. Still fun if you enjoy the competition.
But thinking only of AW, raids and titans, you’ve outgrown 3*. Certainly keep the best until you have a roster of about 40 4*+ heroes (to give you some flexibility in war team composition). But I wouldn’t invest in further 3* development and thus would feed away all unleveled 3*.
Okay, actually I’d hold one of each as I’m a bit of a collector, but I have no plans to level them.