Lee Sin is really strong right now because of the changes to armor pen and stealth. He does have high early game damage, but he falls off pretty hard later in the game to the point where he is just an Ult-Bot. It is hard to deal with him early for many champions simply because they don't have the same level of power in the first 10 minutes compared to him. But that doesn't make him worthy of nerfs. I see more Lee Sin players struggle to be effective these days compared to season 4.
Hecarim is in the same ballpark as Lee, but instead it was the changes to Trinity Force and Ghost and the addition of CotC that made him stronger. He doesn't fall off as hard as Lee, but he doesn't have the damage to really do anything against a coordinated team in a late-game teamfight. Nerfs to CotC will put him in a good spot.
Zac complaints actually confuse me. If he fails a single gank he falls way behind and his damage after the 15 minute mark is abysmal unless he goes with AP items, and if he goes with an AP item or two he is sacrificing tank stats, making him easier to kill. And if you see Zac on the other team just pick Alistar or Thresh. They can completely prevent him from hitting your team. He isn't nearly as bad as he was in season 3. He is impactful, sure, but I don't think nerfing him is the right call. CotC nerfs will fix him, for the most part.
All 3 of these guys also really benefit from the experience cheese that you get from Krugs more so than most other junglers. Once they fix that I think a lot of the problems with them will go away.